# Difference between revisions of "Being Bayesian about Categorical Probability"

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== Introduction == | == Introduction == | ||

− | Softmax (Bridle, 1990) is a staple for neural network’s performing classification--it exponentiates each logit then normalizes by the sum, giving a distribution over the target classes. However, networks with softmax outputs give no information about uncertainty (Blundell et al., 2015; Gal & Ghahramani, 2016), and the resulting distribution over classes is poorly calibrated (Guo et al., 2017), often giving overconfident predictions even when the classification is wrong. Bayesian Neural Networks (BNNs; MacKay, 1992) can alleviate these issues, but the resulting posteriors over the parameters are often intractable. Approximations such as variational inference (Graves, 2011; Blundell et al., 2015) and Monte Carlo Dropout (Gal & Ghahramani, 2016) can still be expensive or give poor estimates for the posteriors. This work proposes a Bayesian treatment of the output logits of the neural network, treating the targets as a categorical random variable instead of a fixed label. This gives a computationally cheap way to get well-calibrated uncertainty estimates on neural network classifications. | + | Since the outputs of neural networks are not probabilities, Softmax (Bridle, 1990) is a staple for neural network’s performing classification -- it exponentiates each logit then normalizes by the sum, giving a distribution over the target classes. Logit is a raw output/prediction of the model which is hard for humans to interpret, thus we transform/normalize these raw values into categories or meaningful numbers for interpretability. However, networks with softmax outputs give no information about uncertainty (Blundell et al., 2015; Gal & Ghahramani, 2016), and the resulting distribution over classes is poorly calibrated (Guo et al., 2017), often giving overconfident predictions even when the classification is wrong. In addition, softmax also raises concerns about overfitting NNs due to its confident predictive behaviors (Xie et al., 2016; Pereyra et al., 2017). To achieve performance with better generalization, some more effective regularization techniques might be required. |

+ | |||

+ | Bayesian Neural Networks (BNNs; MacKay, 1992) can alleviate these issues, but the resulting posteriors over the parameters are often intractable. Approximations such as variational inference (Graves, 2011; Blundell et al., 2015) and Monte Carlo Dropout (Gal & Ghahramani, 2016) can still be expensive or give poor estimates for the posteriors. This work proposes a Bayesian treatment of the output logits of the neural network, treating the targets as a categorical random variable instead of a fixed label. This technique gives a computationally cheap way of being Bayesian to get well-calibrated uncertainty estimates on neural network classifications. | ||

== Related Work == | == Related Work == | ||

− | Using Bayesian Neural Networks is the dominant way of applying Bayesian techniques to neural networks. Many techniques have been developed to make posterior approximation more accurate and scalable, | + | Using Bayesian Neural Networks is the dominant way of applying Bayesian techniques to neural networks. A Bayesian neural network is a stochastic artificial neural network that is trained using Bayesian inference. Bayesian neural networks usually have better calibration than classical neural networks, which indicates that their predicted uncertainty is more consistent with the observed errors. Bayesian networks are data-efficient and can learn with small datasets without overfitting (Jospin, Buntine, Boussaid, Laga, & Bennamoun, 2020). Many techniques have been developed to make posterior approximation more accurate and scalable, despite these, BNNs do not scale to the state of the art techniques or large data sets. There are techniques to explicitly avoid modeling the full weight posterior that is more scalable, such as with Monte Carlo Dropout (Gal & Ghahramani, 2016) or tracking mean/covariance of the posterior during training (Mandt et al., 2017; Zhang et al., 2018; Maddox et al., 2019; Osawa et al., 2019). Non-Bayesian uncertainty estimation techniques such as deep ensembles (Lakshminarayanan et al., 2017) and temperature scaling (Guo et al., 2017; Neumann et al., 2018). |

− | + | ||

+ | == Preliminaries == | ||

+ | === Definitions === | ||

+ | Let's formalize our classification problem and define some notations for the rest of this summary: | ||

+ | |||

+ | ::Dataset: | ||

+ | $$ \mathcal D = \{(x_i,y_i)\} \in (\mathcal X \times \mathcal Y)^N $$ | ||

+ | ::General classification model | ||

+ | $$ f^W: \mathcal X \to \mathbb R^K $$ | ||

+ | ::Softmax function: | ||

+ | $$ \phi(x): \mathbb R^K \to [0,1]^K \;\;|\;\; \phi_k(X) = \frac{\exp(f_k^W(x))}{\sum_{k \in K} \exp(f_k^W(x))} $$ | ||

+ | ::Softmax activated NN: | ||

+ | $$ \phi \;\circ\; f^W: \chi \to \Delta^{K-1} $$ | ||

+ | ::NN as a true classifier: | ||

+ | $$ arg\max_i \;\circ\; \phi_i \;\circ\; f^W \;:\; \mathcal X \to \mathcal Y $$ | ||

+ | |||

+ | We'll also define the '''count function''' - a <math>K</math>-vector valued function that outputs the occurences of each class coincident with <math>x</math>: | ||

+ | $$ c^{\mathcal D}(x) = \sum_{(x',y') \in \mathcal D} \mathbb y' I(x' = x) $$ | ||

+ | |||

+ | === Classification With a Neural Network === | ||

+ | A typical loss function used in classification is cross-entropy, which is defined by | ||

+ | |||

+ | $$ l_{\rm CE}(\tilde{y},\phi(f^{W}(x)))=-\sum_k \tilde{y_k} \log \phi_k(f^{W}(x))) $$ | ||

+ | |||

+ | ,here <math>y_k</math> and <math>\phi_k</math> refers to the actual and predicted categorical distribution for each class. It's well known that optimizing <math>f^W</math> for <math>l_{CE}</math> is equivalent to optimizing for <math>l_{KL}</math>, the <math>KL</math> divergence between the true distribution and the distribution modeled by NN, that is: | ||

+ | $$ l_{KL}(W) = KL(\text{true distribution} \;|\; \text{distribution encoded by }NN(W)) $$ | ||

+ | Let's introduce notations for the underlying (true) distributions of our problem. Let <math>(x_0,y_0) \sim (\mathcal X \times \mathcal Y)</math>: | ||

+ | $$ \text{Full Distribution} = F(x,y) = P(x_0 = x,y_0 = y) $$ | ||

+ | $$ \text{Marginal Distribution} = P(x) = F(x_0 = x) $$ | ||

+ | $$ \text{Point Class Distribution} = P(y_0 = y \;|\; x_0 = x) = F_x(y) $$ | ||

+ | Then we have the following factorization: | ||

+ | $$ F(x,y) = P(x,y) = P(y|x)P(x) = F_x(y)F(x) $$ | ||

+ | Substitute this into the definition of KL divergence: | ||

+ | $$ = \sum_{(x,y) \in \mathcal X \times \mathcal Y} F(x,y) \log\left(\frac{F(x,y)}{\phi_y(f^W(x))}\right) $$ | ||

+ | $$ = \sum_{x \in \mathcal X} F(x) \sum_{y \in \mathcal Y} F(y|x) \log\left( \frac{F(y|x)}{\phi_y(f^W(x))} \right) $$ | ||

+ | $$ = \sum_{x \in \mathcal X} F(x) \sum_{y \in \mathcal Y} F_x(y) \log\left( \frac{F_x(y)}{\phi_y(f^W(x))} \right) $$ | ||

+ | $$ = \sum_{x \in \mathcal X} F(x) KL(F_x \;||\; \phi\left( f^W(x) \right)) $$ | ||

+ | As usual, we don't have an analytic form for <math>l</math> (if we did, this would imply we know <math>F_X</math> meaning we knew the distribution in the first place). Instead, estimate from <math>\mathcal D</math>: | ||

+ | $$ F(x) \approx \hat F(x) = \frac{||c^{\mathcal D}(x)||_1}{N} $$ | ||

+ | $$ F_x(y) \approx \hat F_x(y) = \frac{c^{\mathcal D}(x)}{|| c^{\mathcal D}(x) ||_1}$$ | ||

+ | $$ \to l_{KL}(W) = \sum_{x \in \mathcal D} \frac{||c^{\mathcal D}(x)||_1}{N} KL \left( \frac{c^{\mathcal D}(x)}{||c^{\mathcal D}(x)||_1} \;||\; \phi(f^W(x)) \right) $$ | ||

+ | The approximations <math>\hat F, \hat F_X</math> are often not very good though: consider a typical classification such as MNIST, we would never expect two handwritten digits to produce the exact same image. Hence <math>c^{\mathcal D}(x)</math> is (almost) always going to have a single index 1 and the rest 0. This has implications for our approximations: | ||

+ | $$ \hat F(x) \text{ is uniform for all } x \in \mathcal D $$ | ||

+ | $$ \hat F_x(y) \text{ is degenerate for all } x \in \mathcal D $$ | ||

+ | This clearly has implications for overfitting: to minimize the KL term in <math>l_{KL}(W)</math> we want <math>\phi(f^W(x))</math> to be very close to <math>\hat F_x(y)</math> at each point - this means that the loss function is in fact encouraging the neural network to output near degenerate distributions! | ||

+ | |||

+ | '''Label Smoothing''' | ||

+ | |||

+ | One form of regularization to help this problem is called label smoothing. Instead of using the degenerate $$F_x(y)$$ as a target function, let's "smooth" it (by adding a scaled uniform distribution to it) so it's no longer degenerate: | ||

+ | $$ F'_x(y) = (1-\lambda)\hat F_x(y) + \frac \lambda K \vec 1 $$ | ||

+ | |||

+ | '''BNNs''' | ||

+ | |||

+ | BBNs balances the complexity of the model and the distance to target distribution without choosing a single beset configuration (one-hot encoding). Specifically, BNNs with the Gaussian Weight prior $$F_x(y) = N (0,T^{-1} I)$$ has score of configuration <math>W</math> measured by the posterior density $$p_W(W|D) = p(D|W)p_W(W), \log(p_W(W)) = T||W||^2_2$$ | ||

+ | Here <math>||W||^2_2</math> could be a poor proxy to penalized for the model complexity due to its linear nature. | ||

== Method == | == Method == | ||

+ | The main technical proposal of the paper is a Bayesian framework to estimate the (former) target distribution <math>F_x(y)</math>. That is, we construct a posterior distribution of <math> F_x(y) </math> and use that as our new target distribution. We call it the ''belief matching'' (BM) framework. | ||

+ | |||

+ | === Constructing Target Distribution === | ||

+ | Recall that <math>F_x(y)</math> is a k-categorical probability distribution - its PMF can be fully characterized by k numbers that sum to 1. Hence we can encode any such <math>F_x</math> as a point in <math>\Delta^{k-1}</math>. We'll do exactly that - let's call this vector <math>z</math>: | ||

+ | $$ z \in \Delta^{k-1} $$ | ||

+ | $$ \text{prior} = p_{z|x}(z) $$ | ||

+ | $$ \text{conditional} = p_{y|z,x}(y) $$ | ||

+ | $$ \text{posterior} = p_{z|x,y}(z) $$ | ||

+ | Then if we perform inference: | ||

+ | $$ p_{z|x,y}(z) \propto p_{z|x}(z)p_{y|z,x}(y) $$ | ||

+ | The distribution chosen to model prior was <math>dir_K(\beta)</math>: | ||

+ | $$ p_{z|x}(z) = \frac{\Gamma(||\beta||_1)}{\prod_{k=1}^K \Gamma(\beta_k)} \prod_{k=1}^K z_k^{\beta_k - 1} $$ | ||

+ | Note that by definition of <math>z</math>: <math> p_{y|x,z} = z_y </math>. Since the Dirichlet is a conjugate prior to categorical distributions we have a convenient form for the mean of the posterior: | ||

+ | $$ \bar{p_{z|x,y}}(z) = \frac{\beta + c^{\mathcal D}(x)}{||\beta + c^{\mathcal D}(x)||_1} \propto \beta + c^{\mathcal D}(x) $$ | ||

+ | This is in fact a generalization of (uniform) label smoothing (label smoothing is a special case where <math>\beta = \frac 1 K \vec{1} </math>). | ||

+ | |||

+ | === Representing Approximate Distribution === | ||

+ | Our new target distribution is <math>p_{z|x,y}(z)</math> (as opposed to <math>F_x(y)</math>). That is, we want to construct an interpretation of our neural network weights to construct a distribution with support in <math> \Delta^{K-1} </math> - the NN can then be trained so this encoded distribution closely approximates <math>p_{z|x,y}</math>. Let's denote the PMF of this encoded distribution <math>q_{z|x}^W</math>. This is how the BM framework defines it: | ||

+ | $$ \alpha^W(x) := \exp(f^W(x)) $$ | ||

+ | $$ q_{z|x}^W(z) = \frac{\Gamma(||\alpha^W(x)||_1)}{\sum_{k=1}^K \Gamma(\alpha_k^W(x))} \prod_{k=1}^K z_{k}^{\alpha_k^W(x) - 1} $$ | ||

+ | $$ \to Z^W_x \sim dir(\alpha^W(x)) $$ | ||

+ | Apply <math>\log</math> then <math>\exp</math> to <math>q_{z|x}^W</math>: | ||

+ | $$ q^W_{z|x}(z) \propto \exp \left( \sum_k (\alpha_k^W(x) \log(z_k)) - \sum_k \log(z_k) \right) $$ | ||

+ | $$ \propto -l_{CE}(\phi(f^W(x)),z) + \frac{K}{||\alpha^W(x)||}KL(\mathcal U_k \;||\; z) $$ | ||

+ | It can actually be shown that the mean of <math>Z_x^W</math> is identical to <math>\phi(f^W(x))</math> - in other words, if we output the mean of the encoded distribution of our neural network under the BM framework, it is theoretically identical to a traditional neural network. | ||

+ | In the limit of <math> q^W_{z|x}(z) \rightarrow p_{z|x}(z)</math>, mean of the target posterior becomes a virtual label, for which individual z ought to match. Hence, the penalty for ambiguous configuration is determined by the number of observations. Therefore, the distribution matching in BM can be thought of as '''learning to score a categorical probability''' based on the closeness of the posterior mean, in which exploitation on the closeness of information is automatically controlled by the data. | ||

=== Distribution Matching === | === Distribution Matching === | ||

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=== On Prior Distributions === | === On Prior Distributions === | ||

− | We must choose our concentration parameter, | + | We must choose our concentration parameter, <math>\beta</math>, for our dirichlet prior. We see our prior essentially disappears as <math>\beta_0 \to 0</math> and becomes stronger as <math>\beta_0 \to \infty</math>. Thus, we want a small <math>\beta_0</math> so the posterior isn't dominated by the prior. But, the authors claim that a small <math>\beta_0</math> makes <math>\alpha_0^{\mathbf W}(\mathbf x)</math> small, which causes <math>\psi (\alpha_0^{\mathbf W}(\mathbf x))</math> to be large, which is problematic for gradient based optimization. In practice, many neural network techniques aim to make <math>\mathbb E [f^{\mathbf W} (\mathbf x)] \approx \mathbf 0</math> and thus <math>\mathbb E [\alpha^{\mathbf W} (\mathbf x)] \approx \mathbf 1</math>, which means making <math>\alpha_0^{\mathbf W}(\mathbf x)</math> small can be counterproductive. |

So, the authors set <math>\beta = \mathbf 1</math> and introduce a new hyperparameter <math>\lambda</math> which is multiplied with the KL term in the ELBO: | So, the authors set <math>\beta = \mathbf 1</math> and introduce a new hyperparameter <math>\lambda</math> which is multiplied with the KL term in the ELBO: | ||

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-\left(1-\left(\tilde{\alpha}_{0}^{W}(\mathbf{x})-\lambda K\right)\right)$$ | -\left(1-\left(\tilde{\alpha}_{0}^{W}(\mathbf{x})-\lambda K\right)\right)$$ | ||

− | As we can see, the first expression is affected by the magnitude of | + | As we can see, the first expression is affected by the magnitude of <math>\alpha^{\boldsymbol{W}}(\boldsymbol{x})</math>, whereas the second expression is not due to the <math>\frac{\psi^{\prime}\left(\tilde{\alpha}_{k}^{\mathbf W}(\mathbf{x})\right)}{\psi^{\prime}\left(\tilde{\alpha}_{0}^{\mathbf W}(\mathbf{x})\right)}</math> ratio. |

== Experiments == | == Experiments == | ||

− | Throughout the experiments in this paper, the authors employ various models based on residual connections (He et al., 2016 [1]) which are the models used for benchmarking in practice. The only additions in the experiments are initial learning rate warm-up and gradient clipping which are extremely helpful for stable training of BM. | + | Throughout the experiments in this paper, the authors employ various models based on residual connections (He et al., 2016 [1]) which are the models used for benchmarking in practice. We will first demonstrate improvements provided by BM, then we will show versatility in other applications. For fairness of comparisons, all configurations in the reference implementation will be fixed. The only additions in the experiments are initial learning rate warm-up and gradient clipping which are extremely helpful for stable training of BM. |

=== Generalization performance === | === Generalization performance === | ||

The paper compares the generalization performance of BM with softmax and MC dropout on CIFAR-10 and CIFAR-100 benchmarks. | The paper compares the generalization performance of BM with softmax and MC dropout on CIFAR-10 and CIFAR-100 benchmarks. | ||

+ | |||

+ | [[File:Being_Bayesian_about_Categorical_Probability_T1.png]] | ||

The next comparison was performed between BM and softmax on the ImageNet benchmark. | The next comparison was performed between BM and softmax on the ImageNet benchmark. | ||

+ | |||

+ | [[File:Being_Bayesian_about_Categorical_Probability_T2.png]] | ||

For both datasets and In all configurations, BM achieves the best generalization and outperforms softmax and MC dropout. | For both datasets and In all configurations, BM achieves the best generalization and outperforms softmax and MC dropout. | ||

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===== Impact of <math>\beta</math> ===== | ===== Impact of <math>\beta</math> ===== | ||

− | The effect of β on generalization performance is studied by training ResNet-18 on CIFAR-10 by tuning the value of β on its own, as well as jointly with λ. It was found that robust generalization performance is obtained for β ∈ [ | + | The effect of β on generalization performance is studied by training ResNet-18 on CIFAR-10 by tuning the value of β on its own, as well as jointly with λ. It was found that robust generalization performance is obtained for β ∈ [<math>e^{−1}, e^4</math>] when tuning β on its own; and β ∈ [<math>e^{−4}, e^{8}</math>] when tuning β jointly with λ. The figure below shows a plot of the error rate with varying β. |

+ | |||

+ | [[File:Being_Bayesian_about_Categorical_Probability_F3.png]] | ||

=== Uncertainty Representation === | === Uncertainty Representation === | ||

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===== ID uncertainty ===== | ===== ID uncertainty ===== | ||

− | For ID (in-distribution) samples, calibration performance is measured, which is a measure of how well the model’s confidence matches its actual accuracy. This measure can be visualized using reliability plots and quantified using a metric called expected calibration error (ECE). ECE is calculated by grouping predictions into M groups based on their confidence score and then finding the absolute difference between the average accuracy and average confidence for each group. | + | For ID (in-distribution) samples, calibration performance is measured, which is a measure of how well the model’s confidence matches its actual accuracy. This measure can be visualized using reliability plots and quantified using a metric called expected calibration error (ECE). ECE is calculated by grouping predictions into M groups based on their confidence score and then finding the absolute difference between the average accuracy and average confidence for each group. We can define the ECE of <math>f^W </math> on <math>D </math> with <math>M</math> groups as |

+ | |||

+ | <center> | ||

+ | <math>ECE_M(f^W, D) = \sum^M_{i=1} \frac{|G_i|}{|D|}|acc(G_i) - conf(G_i)|</math> | ||

+ | </center> | ||

+ | Where <math>G_i</math> is a set of samples int the i-th group defined as <math>G_i = \{j:i/M < max_k\phi_k(f^Wx^{(j)}) \leq (1+i)/M\}</math>, <math>acc(G_i)</math> is an average accuracy in the i-th group and <math>conf(G_i)</math> is an average confidence in the i-th group. | ||

+ | |||

The figure below is a reliability plot of ResNet-50 on CIFAR-10 and CIFAR-100 with 15 groups. It shows that BM has a significantly better calibration performance than softmax since the confidence matches the accuracy more closely (this is also reflected in the lower ECE). | The figure below is a reliability plot of ResNet-50 on CIFAR-10 and CIFAR-100 with 15 groups. It shows that BM has a significantly better calibration performance than softmax since the confidence matches the accuracy more closely (this is also reflected in the lower ECE). | ||

+ | |||

+ | [[File:Being_Bayesian_about_Categorical_Probability_F4.png]] | ||

===== OOD uncertainty ===== | ===== OOD uncertainty ===== | ||

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The figure below is a density plot of the predictive entropy of ResNet-50 on CIFAR-10. It shows that BM provides significantly better uncertainty estimation compared to other methods since BM is the only method that has a clear peak of high predictive entropy for OOD samples which should have high uncertainty. | The figure below is a density plot of the predictive entropy of ResNet-50 on CIFAR-10. It shows that BM provides significantly better uncertainty estimation compared to other methods since BM is the only method that has a clear peak of high predictive entropy for OOD samples which should have high uncertainty. | ||

+ | |||

+ | [[File:Being_Bayesian_about_Categorical_Probability_F5.png]] | ||

=== Transfer learning === | === Transfer learning === | ||

− | Belief matching applies the Bayesian principle outside the neural network, which means it can easily be applied to already trained models. Thus, belief matching can be employed in transfer learning scenarios. The authors downloaded the ImageNet | + | Belief matching applies the Bayesian principle outside the neural network, which means it can easily be applied to already trained models. Thus, belief matching can be employed in transfer learning scenarios. The authors downloaded the ImageNet pre-trained ResNet-50 weights and fine-tuned the weights of the last linear layer for 100 epochs using an Adam optimizer. |

This table shows the test error rates from transfer learning on CIFAR-10, Food-101, and Cars datasets. Belief matching consistently performs better than softmax. | This table shows the test error rates from transfer learning on CIFAR-10, Food-101, and Cars datasets. Belief matching consistently performs better than softmax. | ||

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[[File:being_bayesian_about_categorical_probability_transfer_learning.png]] | [[File:being_bayesian_about_categorical_probability_transfer_learning.png]] | ||

− | Belief matching was also tested for the predictive uncertainty for out of dataset samples based on CIFAR-10 as the in distribution sample. Looking at the figure below, it is observed that belief matching significantly improves the uncertainty representation of | + | Belief matching was also tested for the predictive uncertainty for out of dataset samples based on CIFAR-10 as the in distribution sample. Looking at the figure below, it is observed that belief matching significantly improves the uncertainty representation of pre-trained models by only fine-tuning the last layer’s weights. Note that belief matching confidently predicts examples in Cars since CIFAR-10 contains the object category automobiles. In comparison, softmax produces confident predictions on all datasets. Thus, belief matching could also be used to enhance the uncertainty representation ability of pre-trained models without sacrificing their generalization performance. |

[[File: being_bayesian_about_categorical_probability_transfer_learning_uncertainty.png]] | [[File: being_bayesian_about_categorical_probability_transfer_learning_uncertainty.png]] | ||

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=== Semi-Supervised Learning === | === Semi-Supervised Learning === | ||

− | Belief matching’s ability to allow neural networks to represent rich information in their predictions can be exploited to aid consistency based loss function for semi-supervised learning. Consistency-based loss functions use unlabelled samples to determine where to promote robustness of predictions based on stochastic perturbations. This can be done by perturbing the inputs (which is the VAT model) or the networks (which is the pi-model). Both methods minimize the divergence between two categorical probabilities under some perturbations, thus belief matching can be used by the following replacements in the loss functions. The hope is that belief matching can provide better prediction consistencies using its Dirichlet distributions. | + | Belief matching’s ability to allow neural networks to represent rich information in their predictions can be exploited to aid consistency based loss function for semi-supervised learning. Consistency-based loss functions use unlabelled samples to determine where to promote the robustness of predictions based on stochastic perturbations. This can be done by perturbing the inputs (which is the VAT model) or the networks (which is the pi-model). Both methods minimize the divergence between two categorical probabilities under some perturbations, thus belief matching can be used by the following replacements in the loss functions. The hope is that belief matching can provide better prediction consistencies using its Dirichlet distributions. |

[[File: being_bayesian_about_categorical_probability_semi_supervised_equation.png]] | [[File: being_bayesian_about_categorical_probability_semi_supervised_equation.png]] | ||

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[[File:being_bayesian_about_categorical_probability_semi_supervised_table.png]] | [[File:being_bayesian_about_categorical_probability_semi_supervised_table.png]] | ||

− | == Conclusion == | + | == Conclusion and Critiques == |

− | Bayesian principles can be used to construct the target distribution by using the categorical probability as a random variable rather than a training label. This can be applied to neural network models by replacing only the softmax and cross-entropy loss | + | * Bayesian principles can be used to construct the target distribution by using the categorical probability as a random variable rather than a training label. This can be applied to neural network models by replacing only the softmax and cross-entropy loss while improving the generalization performance, uncertainty estimation and well-calibrated behavior. |

− | In the future, the authors would like to allow for more expressive distributions in the belief matching framework, such as logistic normal distributions to capture strong semantic similarities among class labels. Furthermore, using input dependent priors would allow for interesting properties that would aid imbalanced datasets and multi-domain learning. | + | * In the future, the authors would like to allow for more expressive distributions in the belief matching framework, such as logistic normal distributions to capture strong semantic similarities among class labels. Furthermore, using input dependent priors would allow for interesting properties that would aid imbalanced datasets and multi-domain learning. |

+ | |||

+ | * Overall I think this summary is very good. The Method(Algorithm) section is described clearly, and the Results section is detailed, with many diagrams illustrating the main points. I just have one technical suggestion: the difference in performance for SOFTMAX and BM differs by model. For example, for RESNEXT-50 model, the difference in top1 is 0.2, whereas, for the RESNEXT-100 model, the difference in the top one is 0.5, which is significantly higher. It's true that BM method generally outperforms SOFTMAX. But seeing the relation between the choice of model and the magnitude of a performance increase could definitely strengthen the paper even further. | ||

+ | |||

+ | * The summary is good and the topic is interesting. Bayesian is a well know probabilistic model but did not know that it can be used as a neural network. Comparison between softmax and bayesian was interesting and more details would be great. | ||

+ | |||

+ | * It would be better if there is a future work section to discuss the current shortage and potential improvement. One thing would be that the theoretical part is complex in the process. In addition, optimizing a function is relatively hard if the structure is complex. Is it possible to have a good approximation without having too complex a calculation? | ||

+ | |||

+ | * Both experiments dealt with image data, however, softmax is used within classification neural networks that range from image to textual data. It would be interesting to see the performance of BM on textual data for text classification problems in addition to image classification. | ||

+ | |||

+ | * It would be better to briefly explain Bayesian treatment in the introduction part(i.e., considering the categorical probability as a random variable, construct the target distribution by means of the Bayesian inference), and to analyze the importance of considering the categorical probability as a random variable (for example explain it can be adapted to existing deep learning building blocks without huge modifications). | ||

+ | |||

+ | * Interesting topic that goes close to our lectures. Since this is a summary of the paper, it would be better if trim the explanation on Neural Network a little like getting rid of the substitution lines. | ||

+ | |||

+ | * I really liked the presentation and actually really appreciate the steps of the detailed derivation that were presented in this summary. In the introduction the researchers mentioned that BM is a computationally cheap method, however, I was wondering how much faster it is computationally as opposed to the other models to train. Additionally, the training data that was used to benchmark the classification performance seemed to all be image classifications (CIFAR-10, CIFAR-100, ResNet-50, ResNet-101), thus it would have been nice to see classification be applied in other multi-class contexts as well to see how well this new method performs there. | ||

+ | |||

+ | * It would be more clear if the metric used in evaluating the models is briefly explained! | ||

+ | |||

+ | * The author's work by applying a Bayesian treatment of the output logits of the neural network to reach a more effective regularization technique is impressive. However, the author lacks a comparison of the calculation efficiency difference between the results. When dealing with large data sets, it is difficult to be convincing if there is no advantage in computational efficiency. | ||

+ | |||

+ | * The paper was well written with few flaws, data was meticulously presented, yet with ensemble networks, it is important to notify the computational complexity increase that comes with training multiple models. It would also be nice to see their loss and entropy with regards to a standard training process (same epoch, train test split etc). | ||

+ | |||

+ | * In the original paper it contains a figure 2 of Penultimate layer’s activations of examples belonging to one of three classes (beaver, dolphin, and otter; indexed by 0,1,2 in CIFAR-100). The figure vividly shows how successful the result is, so it would be better contained in the summary. | ||

== Citations == | == Citations == | ||

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[12] Neumann, L., Zisserman, A., and Vedaldi, A. Relaxed softmax: Efficient confidence auto-calibration for safe pedestrian detection. In NIPS Workshop on Machine Learning for Intelligent Transportation Systems, 2018. | [12] Neumann, L., Zisserman, A., and Vedaldi, A. Relaxed softmax: Efficient confidence auto-calibration for safe pedestrian detection. In NIPS Workshop on Machine Learning for Intelligent Transportation Systems, 2018. | ||

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+ | [13] Xie, L., Wang, J., Wei, Z., Wang, M., and Tian, Q. Disturblabel: Regularizing cnn on the loss layer. In IEEE Conference on Computer Vision and Pattern Recognition, 2016. | ||

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+ | [14] Pereyra, G., Tucker, G., Chorowski, J., Kaiser, Ł., and Hinton, G. Regularizing neural networks by penalizing confident output distributions. arXiv preprint arXiv:1701.06548, 2017. | ||

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+ | [15] Jospin, L. V., Buntine, W. V., Boussaid, F. V., Laga, H. V., & Bennamoun, M. V. (2020). Hands-on Bayesian Neural Networks - a Tutorial for Deep Learning Users. Association for Computing Machiner, 3-7. doi:arXiv:2007.06823 |

## Latest revision as of 09:57, 15 December 2020

## Contents

## Presented By

Evan Li, Jason Pu, Karam Abuaisha, Nicholas Vadivelu

## Introduction

Since the outputs of neural networks are not probabilities, Softmax (Bridle, 1990) is a staple for neural network’s performing classification -- it exponentiates each logit then normalizes by the sum, giving a distribution over the target classes. Logit is a raw output/prediction of the model which is hard for humans to interpret, thus we transform/normalize these raw values into categories or meaningful numbers for interpretability. However, networks with softmax outputs give no information about uncertainty (Blundell et al., 2015; Gal & Ghahramani, 2016), and the resulting distribution over classes is poorly calibrated (Guo et al., 2017), often giving overconfident predictions even when the classification is wrong. In addition, softmax also raises concerns about overfitting NNs due to its confident predictive behaviors (Xie et al., 2016; Pereyra et al., 2017). To achieve performance with better generalization, some more effective regularization techniques might be required.

Bayesian Neural Networks (BNNs; MacKay, 1992) can alleviate these issues, but the resulting posteriors over the parameters are often intractable. Approximations such as variational inference (Graves, 2011; Blundell et al., 2015) and Monte Carlo Dropout (Gal & Ghahramani, 2016) can still be expensive or give poor estimates for the posteriors. This work proposes a Bayesian treatment of the output logits of the neural network, treating the targets as a categorical random variable instead of a fixed label. This technique gives a computationally cheap way of being Bayesian to get well-calibrated uncertainty estimates on neural network classifications.

## Related Work

Using Bayesian Neural Networks is the dominant way of applying Bayesian techniques to neural networks. A Bayesian neural network is a stochastic artificial neural network that is trained using Bayesian inference. Bayesian neural networks usually have better calibration than classical neural networks, which indicates that their predicted uncertainty is more consistent with the observed errors. Bayesian networks are data-efficient and can learn with small datasets without overfitting (Jospin, Buntine, Boussaid, Laga, & Bennamoun, 2020). Many techniques have been developed to make posterior approximation more accurate and scalable, despite these, BNNs do not scale to the state of the art techniques or large data sets. There are techniques to explicitly avoid modeling the full weight posterior that is more scalable, such as with Monte Carlo Dropout (Gal & Ghahramani, 2016) or tracking mean/covariance of the posterior during training (Mandt et al., 2017; Zhang et al., 2018; Maddox et al., 2019; Osawa et al., 2019). Non-Bayesian uncertainty estimation techniques such as deep ensembles (Lakshminarayanan et al., 2017) and temperature scaling (Guo et al., 2017; Neumann et al., 2018).

## Preliminaries

### Definitions

Let's formalize our classification problem and define some notations for the rest of this summary:

- Dataset:

$$ \mathcal D = \{(x_i,y_i)\} \in (\mathcal X \times \mathcal Y)^N $$

- General classification model

$$ f^W: \mathcal X \to \mathbb R^K $$

- Softmax function:

$$ \phi(x): \mathbb R^K \to [0,1]^K \;\;|\;\; \phi_k(X) = \frac{\exp(f_k^W(x))}{\sum_{k \in K} \exp(f_k^W(x))} $$

- Softmax activated NN:

$$ \phi \;\circ\; f^W: \chi \to \Delta^{K-1} $$

- NN as a true classifier:

$$ arg\max_i \;\circ\; \phi_i \;\circ\; f^W \;:\; \mathcal X \to \mathcal Y $$

We'll also define the **count function** - a [math]K[/math]-vector valued function that outputs the occurences of each class coincident with [math]x[/math]:
$$ c^{\mathcal D}(x) = \sum_{(x',y') \in \mathcal D} \mathbb y' I(x' = x) $$

### Classification With a Neural Network

A typical loss function used in classification is cross-entropy, which is defined by

$$ l_{\rm CE}(\tilde{y},\phi(f^{W}(x)))=-\sum_k \tilde{y_k} \log \phi_k(f^{W}(x))) $$

,here [math]y_k[/math] and [math]\phi_k[/math] refers to the actual and predicted categorical distribution for each class. It's well known that optimizing [math]f^W[/math] for [math]l_{CE}[/math] is equivalent to optimizing for [math]l_{KL}[/math], the [math]KL[/math] divergence between the true distribution and the distribution modeled by NN, that is: $$ l_{KL}(W) = KL(\text{true distribution} \;|\; \text{distribution encoded by }NN(W)) $$ Let's introduce notations for the underlying (true) distributions of our problem. Let [math](x_0,y_0) \sim (\mathcal X \times \mathcal Y)[/math]: $$ \text{Full Distribution} = F(x,y) = P(x_0 = x,y_0 = y) $$ $$ \text{Marginal Distribution} = P(x) = F(x_0 = x) $$ $$ \text{Point Class Distribution} = P(y_0 = y \;|\; x_0 = x) = F_x(y) $$ Then we have the following factorization: $$ F(x,y) = P(x,y) = P(y|x)P(x) = F_x(y)F(x) $$ Substitute this into the definition of KL divergence: $$ = \sum_{(x,y) \in \mathcal X \times \mathcal Y} F(x,y) \log\left(\frac{F(x,y)}{\phi_y(f^W(x))}\right) $$ $$ = \sum_{x \in \mathcal X} F(x) \sum_{y \in \mathcal Y} F(y|x) \log\left( \frac{F(y|x)}{\phi_y(f^W(x))} \right) $$ $$ = \sum_{x \in \mathcal X} F(x) \sum_{y \in \mathcal Y} F_x(y) \log\left( \frac{F_x(y)}{\phi_y(f^W(x))} \right) $$ $$ = \sum_{x \in \mathcal X} F(x) KL(F_x \;||\; \phi\left( f^W(x) \right)) $$ As usual, we don't have an analytic form for [math]l[/math] (if we did, this would imply we know [math]F_X[/math] meaning we knew the distribution in the first place). Instead, estimate from [math]\mathcal D[/math]: $$ F(x) \approx \hat F(x) = \frac{||c^{\mathcal D}(x)||_1}{N} $$ $$ F_x(y) \approx \hat F_x(y) = \frac{c^{\mathcal D}(x)}{|| c^{\mathcal D}(x) ||_1}$$ $$ \to l_{KL}(W) = \sum_{x \in \mathcal D} \frac{||c^{\mathcal D}(x)||_1}{N} KL \left( \frac{c^{\mathcal D}(x)}{||c^{\mathcal D}(x)||_1} \;||\; \phi(f^W(x)) \right) $$ The approximations [math]\hat F, \hat F_X[/math] are often not very good though: consider a typical classification such as MNIST, we would never expect two handwritten digits to produce the exact same image. Hence [math]c^{\mathcal D}(x)[/math] is (almost) always going to have a single index 1 and the rest 0. This has implications for our approximations: $$ \hat F(x) \text{ is uniform for all } x \in \mathcal D $$ $$ \hat F_x(y) \text{ is degenerate for all } x \in \mathcal D $$ This clearly has implications for overfitting: to minimize the KL term in [math]l_{KL}(W)[/math] we want [math]\phi(f^W(x))[/math] to be very close to [math]\hat F_x(y)[/math] at each point - this means that the loss function is in fact encouraging the neural network to output near degenerate distributions!

**Label Smoothing**

One form of regularization to help this problem is called label smoothing. Instead of using the degenerate $$F_x(y)$$ as a target function, let's "smooth" it (by adding a scaled uniform distribution to it) so it's no longer degenerate: $$ F'_x(y) = (1-\lambda)\hat F_x(y) + \frac \lambda K \vec 1 $$

**BNNs**

BBNs balances the complexity of the model and the distance to target distribution without choosing a single beset configuration (one-hot encoding). Specifically, BNNs with the Gaussian Weight prior $$F_x(y) = N (0,T^{-1} I)$$ has score of configuration [math]W[/math] measured by the posterior density $$p_W(W|D) = p(D|W)p_W(W), \log(p_W(W)) = T||W||^2_2$$ Here [math]||W||^2_2[/math] could be a poor proxy to penalized for the model complexity due to its linear nature.

## Method

The main technical proposal of the paper is a Bayesian framework to estimate the (former) target distribution [math]F_x(y)[/math]. That is, we construct a posterior distribution of [math] F_x(y) [/math] and use that as our new target distribution. We call it the *belief matching* (BM) framework.

### Constructing Target Distribution

Recall that [math]F_x(y)[/math] is a k-categorical probability distribution - its PMF can be fully characterized by k numbers that sum to 1. Hence we can encode any such [math]F_x[/math] as a point in [math]\Delta^{k-1}[/math]. We'll do exactly that - let's call this vector [math]z[/math]: $$ z \in \Delta^{k-1} $$ $$ \text{prior} = p_{z|x}(z) $$ $$ \text{conditional} = p_{y|z,x}(y) $$ $$ \text{posterior} = p_{z|x,y}(z) $$ Then if we perform inference: $$ p_{z|x,y}(z) \propto p_{z|x}(z)p_{y|z,x}(y) $$ The distribution chosen to model prior was [math]dir_K(\beta)[/math]: $$ p_{z|x}(z) = \frac{\Gamma(||\beta||_1)}{\prod_{k=1}^K \Gamma(\beta_k)} \prod_{k=1}^K z_k^{\beta_k - 1} $$ Note that by definition of [math]z[/math]: [math] p_{y|x,z} = z_y [/math]. Since the Dirichlet is a conjugate prior to categorical distributions we have a convenient form for the mean of the posterior: $$ \bar{p_{z|x,y}}(z) = \frac{\beta + c^{\mathcal D}(x)}{||\beta + c^{\mathcal D}(x)||_1} \propto \beta + c^{\mathcal D}(x) $$ This is in fact a generalization of (uniform) label smoothing (label smoothing is a special case where [math]\beta = \frac 1 K \vec{1} [/math]).

### Representing Approximate Distribution

Our new target distribution is [math]p_{z|x,y}(z)[/math] (as opposed to [math]F_x(y)[/math]). That is, we want to construct an interpretation of our neural network weights to construct a distribution with support in [math] \Delta^{K-1} [/math] - the NN can then be trained so this encoded distribution closely approximates [math]p_{z|x,y}[/math]. Let's denote the PMF of this encoded distribution [math]q_{z|x}^W[/math]. This is how the BM framework defines it: $$ \alpha^W(x) := \exp(f^W(x)) $$ $$ q_{z|x}^W(z) = \frac{\Gamma(||\alpha^W(x)||_1)}{\sum_{k=1}^K \Gamma(\alpha_k^W(x))} \prod_{k=1}^K z_{k}^{\alpha_k^W(x) - 1} $$ $$ \to Z^W_x \sim dir(\alpha^W(x)) $$ Apply [math]\log[/math] then [math]\exp[/math] to [math]q_{z|x}^W[/math]: $$ q^W_{z|x}(z) \propto \exp \left( \sum_k (\alpha_k^W(x) \log(z_k)) - \sum_k \log(z_k) \right) $$ $$ \propto -l_{CE}(\phi(f^W(x)),z) + \frac{K}{||\alpha^W(x)||}KL(\mathcal U_k \;||\; z) $$ It can actually be shown that the mean of [math]Z_x^W[/math] is identical to [math]\phi(f^W(x))[/math] - in other words, if we output the mean of the encoded distribution of our neural network under the BM framework, it is theoretically identical to a traditional neural network.

In the limit of [math] q^W_{z|x}(z) \rightarrow p_{z|x}(z)[/math], mean of the target posterior becomes a virtual label, for which individual z ought to match. Hence, the penalty for ambiguous configuration is determined by the number of observations. Therefore, the distribution matching in BM can be thought of as **learning to score a categorical probability** based on the closeness of the posterior mean, in which exploitation on the closeness of information is automatically controlled by the data.

### Distribution Matching

We now need a way to fit our approximate distribution from our neural network [math]q_{\mathbf{z | x}}^{\mathbf{W}}[/math] to our target distribution [math]p_{\mathbf{z|x},y}[/math]. The authors achieve this by maximizing the evidence lower bound (ELBO):

$$l_{EB}(\mathbf y, \alpha^{\mathbf W}(\mathbf x)) = \mathbb E_{q_{\mathbf{z | x}}^{\mathbf{W}}} \left[\log p(\mathbf {y | x, z})\right] - KL (q_{\mathbf{z | x}}^{\mathbf W} \; || \; p_{\mathbf{z|x}}) $$

Each term can be computed analytically:

$$\mathbb E_{q_{\mathbf{z | x}}^{\mathbf{W}}} \left[\log p(\mathbf {y | x, z})\right] = \mathbb E_{q_{\mathbf{z | x}}^{\mathbf W }} \left[\log z_y \right] = \psi(\alpha_y^{\mathbf W} ( \mathbf x )) - \psi(\alpha_0^{\mathbf W} ( \mathbf x )) $$

Where [math]\psi(\cdot)[/math] represents the digamma function (logarithmic derivative of gamma function). Intuitively, we maximize the probability of the correct label. For the KL term:

$$KL (q_{\mathbf{z | x}}^{\mathbf W} \; || \; p_{\mathbf{z|x}}) = \log \frac{\Gamma(a_0^{\mathbf W}(\mathbf x)) \prod_k \Gamma(\beta_k)}{\prod_k \Gamma(\alpha_k^{\mathbf W}(x)) \Gamma (\beta_0)} + \sum_k (\alpha_k^{\mathbf W}(x)-\beta_k)(\psi(\alpha_k^{\mathbf W}(\mathbf x)) - \psi(\alpha_0^{\mathbf W}(\mathbf x)) $$

In the first term, for intuition, we can ignore [math]\alpha_0[/math] and [math]\beta_0[/math] since those just calibrate the distributions. Otherwise, we want the ratio of the products to be as close to 1 as possible to minimize the KL. In the second term, we want to minimize the difference between each individual [math]\alpha_k[/math] and [math]\beta_k[/math], scaled by the normalized output of the neural network.

This loss function can be used as a drop-in replacement for the standard softmax cross-entropy, as it has an analytic form and the same time complexity as typical softmax-cross entropy with respect to the number of classes ([math]O(K)[/math]).

### On Prior Distributions

We must choose our concentration parameter, [math]\beta[/math], for our dirichlet prior. We see our prior essentially disappears as [math]\beta_0 \to 0[/math] and becomes stronger as [math]\beta_0 \to \infty[/math]. Thus, we want a small [math]\beta_0[/math] so the posterior isn't dominated by the prior. But, the authors claim that a small [math]\beta_0[/math] makes [math]\alpha_0^{\mathbf W}(\mathbf x)[/math] small, which causes [math]\psi (\alpha_0^{\mathbf W}(\mathbf x))[/math] to be large, which is problematic for gradient based optimization. In practice, many neural network techniques aim to make [math]\mathbb E [f^{\mathbf W} (\mathbf x)] \approx \mathbf 0[/math] and thus [math]\mathbb E [\alpha^{\mathbf W} (\mathbf x)] \approx \mathbf 1[/math], which means making [math]\alpha_0^{\mathbf W}(\mathbf x)[/math] small can be counterproductive.

So, the authors set [math]\beta = \mathbf 1[/math] and introduce a new hyperparameter [math]\lambda[/math] which is multiplied with the KL term in the ELBO:

$$l^\lambda_{EB}(\mathbf y, \alpha^{\mathbf W}(\mathbf x)) = \mathbb E_{q_{\mathbf{z | x}}^{\mathbf{W}}} \left[\log p(\mathbf {y | x, z})\right] - \lambda KL (q_{\mathbf{z | x}}^{\mathbf W} \; || \; \mathcal P^D (\mathbf 1)) $$

This stabilizes the optimization, as we can tell from the gradients:

$$\frac{\partial l_{E B}\left(\mathbf{y}, \alpha^{\mathbf W}(\mathbf{x})\right)}{\partial \alpha_{k}^{\mathbf W}(\mathbf {x})}=\left(\tilde{\mathbf{y}}_{k}-\left(\alpha_{k}^{\mathbf W}(\mathbf{x})-\beta_{k}\right)\right) \psi^{\prime}\left(\alpha_{k}^{\mathbf{W}}(\boldsymbol{x})\right) -\left(1-\left(\alpha_{0}^{\boldsymbol{W}}(\boldsymbol{x})-\beta_{0}\right)\right) \psi^{\prime}\left(\alpha_{0}^{\boldsymbol{W}}(\boldsymbol{x})\right)$$

$$\frac{\partial l_{E B}^{\lambda}\left(\mathbf{y}, \alpha^{\mathbf{W}}(\mathbf{x})\right)}{\partial \alpha_{k}^{W}(\mathbf{x})}=\left(\tilde{\mathbf{y}}_{k}-\left(\tilde{\alpha}_{k}^{\mathbf W}(\mathbf{x})-\lambda\right)\right) \frac{\psi^{\prime}\left(\tilde{\alpha}_{k}^{\mathbf W}(\mathbf{x})\right)}{\psi^{\prime}\left(\tilde{\alpha}_{0}^{\mathbf W}(\mathbf{x})\right)} -\left(1-\left(\tilde{\alpha}_{0}^{W}(\mathbf{x})-\lambda K\right)\right)$$

As we can see, the first expression is affected by the magnitude of [math]\alpha^{\boldsymbol{W}}(\boldsymbol{x})[/math], whereas the second expression is not due to the [math]\frac{\psi^{\prime}\left(\tilde{\alpha}_{k}^{\mathbf W}(\mathbf{x})\right)}{\psi^{\prime}\left(\tilde{\alpha}_{0}^{\mathbf W}(\mathbf{x})\right)}[/math] ratio.

## Experiments

Throughout the experiments in this paper, the authors employ various models based on residual connections (He et al., 2016 [1]) which are the models used for benchmarking in practice. We will first demonstrate improvements provided by BM, then we will show versatility in other applications. For fairness of comparisons, all configurations in the reference implementation will be fixed. The only additions in the experiments are initial learning rate warm-up and gradient clipping which are extremely helpful for stable training of BM.

### Generalization performance

The paper compares the generalization performance of BM with softmax and MC dropout on CIFAR-10 and CIFAR-100 benchmarks.

The next comparison was performed between BM and softmax on the ImageNet benchmark.

For both datasets and In all configurations, BM achieves the best generalization and outperforms softmax and MC dropout.

##### Regularization effect of prior

In theory, BM has 2 regularization effects: The prior distribution, which smooths the target posterior Averaging all of the possible categorical probabilities to compute the distribution matching loss The authors perform an ablation study to examine the 2 effects separately - removing the KL term in the ELBO removes the effect of the prior distribution. For ResNet-50 on CIFAR-100 and CIFAR-10 the resulting test error rates were 24.69% and 5.68% respectively.

This demonstrates that both regularization effects are significant since just having one of them improves the generalization performance compared to the softmax baseline, and having both improves the performance even more.

##### Impact of [math]\beta[/math]

The effect of β on generalization performance is studied by training ResNet-18 on CIFAR-10 by tuning the value of β on its own, as well as jointly with λ. It was found that robust generalization performance is obtained for β ∈ [[math]e^{−1}, e^4[/math]] when tuning β on its own; and β ∈ [[math]e^{−4}, e^{8}[/math]] when tuning β jointly with λ. The figure below shows a plot of the error rate with varying β.

### Uncertainty Representation

One of the big advantages of BM is the ability to represent uncertainty about the prediction. The authors evaluate the uncertainty representation on in-distribution (ID) and out-of-distribution (OOD) samples.

##### ID uncertainty

For ID (in-distribution) samples, calibration performance is measured, which is a measure of how well the model’s confidence matches its actual accuracy. This measure can be visualized using reliability plots and quantified using a metric called expected calibration error (ECE). ECE is calculated by grouping predictions into M groups based on their confidence score and then finding the absolute difference between the average accuracy and average confidence for each group. We can define the ECE of [math]f^W [/math] on [math]D [/math] with [math]M[/math] groups as

[math]ECE_M(f^W, D) = \sum^M_{i=1} \frac{|G_i|}{|D|}|acc(G_i) - conf(G_i)|[/math]

Where [math]G_i[/math] is a set of samples int the i-th group defined as [math]G_i = \{j:i/M \lt max_k\phi_k(f^Wx^{(j)}) \leq (1+i)/M\}[/math], [math]acc(G_i)[/math] is an average accuracy in the i-th group and [math]conf(G_i)[/math] is an average confidence in the i-th group.

The figure below is a reliability plot of ResNet-50 on CIFAR-10 and CIFAR-100 with 15 groups. It shows that BM has a significantly better calibration performance than softmax since the confidence matches the accuracy more closely (this is also reflected in the lower ECE).

##### OOD uncertainty

Here, the authors quantify uncertainty using predictive entropy - the larger the predictive entropy, the larger the uncertainty about a prediction.

The figure below is a density plot of the predictive entropy of ResNet-50 on CIFAR-10. It shows that BM provides significantly better uncertainty estimation compared to other methods since BM is the only method that has a clear peak of high predictive entropy for OOD samples which should have high uncertainty.

### Transfer learning

Belief matching applies the Bayesian principle outside the neural network, which means it can easily be applied to already trained models. Thus, belief matching can be employed in transfer learning scenarios. The authors downloaded the ImageNet pre-trained ResNet-50 weights and fine-tuned the weights of the last linear layer for 100 epochs using an Adam optimizer.

This table shows the test error rates from transfer learning on CIFAR-10, Food-101, and Cars datasets. Belief matching consistently performs better than softmax.

Belief matching was also tested for the predictive uncertainty for out of dataset samples based on CIFAR-10 as the in distribution sample. Looking at the figure below, it is observed that belief matching significantly improves the uncertainty representation of pre-trained models by only fine-tuning the last layer’s weights. Note that belief matching confidently predicts examples in Cars since CIFAR-10 contains the object category automobiles. In comparison, softmax produces confident predictions on all datasets. Thus, belief matching could also be used to enhance the uncertainty representation ability of pre-trained models without sacrificing their generalization performance.

### Semi-Supervised Learning

Belief matching’s ability to allow neural networks to represent rich information in their predictions can be exploited to aid consistency based loss function for semi-supervised learning. Consistency-based loss functions use unlabelled samples to determine where to promote the robustness of predictions based on stochastic perturbations. This can be done by perturbing the inputs (which is the VAT model) or the networks (which is the pi-model). Both methods minimize the divergence between two categorical probabilities under some perturbations, thus belief matching can be used by the following replacements in the loss functions. The hope is that belief matching can provide better prediction consistencies using its Dirichlet distributions.

The results of training on ResNet28-2 with consistency based loss functions on CIFAR-10 are shown in this table. Belief matching does have lower classification error rates compared to using a softmax.

## Conclusion and Critiques

- Bayesian principles can be used to construct the target distribution by using the categorical probability as a random variable rather than a training label. This can be applied to neural network models by replacing only the softmax and cross-entropy loss while improving the generalization performance, uncertainty estimation and well-calibrated behavior.

- In the future, the authors would like to allow for more expressive distributions in the belief matching framework, such as logistic normal distributions to capture strong semantic similarities among class labels. Furthermore, using input dependent priors would allow for interesting properties that would aid imbalanced datasets and multi-domain learning.

- Overall I think this summary is very good. The Method(Algorithm) section is described clearly, and the Results section is detailed, with many diagrams illustrating the main points. I just have one technical suggestion: the difference in performance for SOFTMAX and BM differs by model. For example, for RESNEXT-50 model, the difference in top1 is 0.2, whereas, for the RESNEXT-100 model, the difference in the top one is 0.5, which is significantly higher. It's true that BM method generally outperforms SOFTMAX. But seeing the relation between the choice of model and the magnitude of a performance increase could definitely strengthen the paper even further.

- The summary is good and the topic is interesting. Bayesian is a well know probabilistic model but did not know that it can be used as a neural network. Comparison between softmax and bayesian was interesting and more details would be great.

- It would be better if there is a future work section to discuss the current shortage and potential improvement. One thing would be that the theoretical part is complex in the process. In addition, optimizing a function is relatively hard if the structure is complex. Is it possible to have a good approximation without having too complex a calculation?

- Both experiments dealt with image data, however, softmax is used within classification neural networks that range from image to textual data. It would be interesting to see the performance of BM on textual data for text classification problems in addition to image classification.

- It would be better to briefly explain Bayesian treatment in the introduction part(i.e., considering the categorical probability as a random variable, construct the target distribution by means of the Bayesian inference), and to analyze the importance of considering the categorical probability as a random variable (for example explain it can be adapted to existing deep learning building blocks without huge modifications).

- Interesting topic that goes close to our lectures. Since this is a summary of the paper, it would be better if trim the explanation on Neural Network a little like getting rid of the substitution lines.

- I really liked the presentation and actually really appreciate the steps of the detailed derivation that were presented in this summary. In the introduction the researchers mentioned that BM is a computationally cheap method, however, I was wondering how much faster it is computationally as opposed to the other models to train. Additionally, the training data that was used to benchmark the classification performance seemed to all be image classifications (CIFAR-10, CIFAR-100, ResNet-50, ResNet-101), thus it would have been nice to see classification be applied in other multi-class contexts as well to see how well this new method performs there.

- It would be more clear if the metric used in evaluating the models is briefly explained!

- The author's work by applying a Bayesian treatment of the output logits of the neural network to reach a more effective regularization technique is impressive. However, the author lacks a comparison of the calculation efficiency difference between the results. When dealing with large data sets, it is difficult to be convincing if there is no advantage in computational efficiency.

- The paper was well written with few flaws, data was meticulously presented, yet with ensemble networks, it is important to notify the computational complexity increase that comes with training multiple models. It would also be nice to see their loss and entropy with regards to a standard training process (same epoch, train test split etc).

- In the original paper it contains a figure 2 of Penultimate layer’s activations of examples belonging to one of three classes (beaver, dolphin, and otter; indexed by 0,1,2 in CIFAR-100). The figure vividly shows how successful the result is, so it would be better contained in the summary.

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[3] Gal, Y. and Ghahramani, Z. Dropout as a Bayesian approximation: Representing model uncertainty in deep learning. In International Conference on Machine Learning, 2016.

[4] Guo, C., Pleiss, G., Sun, Y., and Weinberger, K. Q. On calibration of modern neural networks. In International Conference on Machine Learning, 2017.

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[10] Osawa, K., Swaroop, S., Jain, A., Eschenhagen, R., Turner, R. E., Yokota, R., and Khan, M. E. Practical deep learning with Bayesian principles. In Advances in Neural Information Processing Systems, 2019.

[11] Lakshminarayanan, B., Pritzel, A., and Blundell, C. Simple and scalable predictive uncertainty estimation using deep ensembles. In Advances in Neural Information Processing Systems, 2017.

[12] Neumann, L., Zisserman, A., and Vedaldi, A. Relaxed softmax: Efficient confidence auto-calibration for safe pedestrian detection. In NIPS Workshop on Machine Learning for Intelligent Transportation Systems, 2018.

[13] Xie, L., Wang, J., Wei, Z., Wang, M., and Tian, Q. Disturblabel: Regularizing cnn on the loss layer. In IEEE Conference on Computer Vision and Pattern Recognition, 2016.

[14] Pereyra, G., Tucker, G., Chorowski, J., Kaiser, Ł., and Hinton, G. Regularizing neural networks by penalizing confident output distributions. arXiv preprint arXiv:1701.06548, 2017.

[15] Jospin, L. V., Buntine, W. V., Boussaid, F. V., Laga, H. V., & Bennamoun, M. V. (2020). Hands-on Bayesian Neural Networks - a Tutorial for Deep Learning Users. Association for Computing Machiner, 3-7. doi:arXiv:2007.06823