Research Papers Classification System

Presented by

Jill Wang, Junyi Yang, Yu Min Wu, Chun Kit (Calvin) Li

Introduction

This paper introduces a paper classification system that utilizes the Term frequency-inverse document frequency (TF-IDF), Latent Dirichlet Allocation (LDA), and K-means clustering. The most important technology the system used to process big data is the Hadoop Distributed File Systems (HDFS). The system can handle quantitatively complex research paper classification problems efficiently and accurately.

General Framework

Data Preprocessing

Crawling of Abstract Data

Under the assumption that audiences tend to first read the abstract of papers to gain an understanding of the papers. As a result, the abstract of any paper may include “core words” that can be used to effectively classify papers’ subjects.

An abstract is crawled to have its stop words removed. Stop words are words that are usually ignored by search engines, such as “the”, “a”, and etc. Afterwards, nouns are extracted, as a more condensed representation for efficient analysis.

Managing Paper Data

To construct an effective keyword dictionary using abstract data and keywords data in all of the crawled papers, the authors categorized keywords with similar meanings using a single representative keyword. The approach is called stemming, which is common in cleaning data. 1394 keyword categories are extracted, which is still too much to compute. Hence, only the top 30 keyword categories are used.

Topic Modeling Using LDA

Latent Dirichlet allocation (LDA) is a generative probabilistic model that views documents as random mixtures over latent topics. Each topic is a distribution over words, and the goal is to extract these topics from documents.

LDA estimates topic distribution given a document using Dirichlet priors for the distributions with a fixed number of topics. For each document, obtain a feature vector:

[math]\displaystyle{ F = \left( P\left(z_1 | d\right), P\left(z_2 | d\right), \cdots, P\left(z_k | d\right) \right) }[/math]

In the paper, authors extract topics from preprossed paper to generate three kinds of topic sets, each with 10, 20, and 30 topics respectively. The following is a table of the 10 topic sets of highest frequency keywords.

Term Frequency Inverse Document Frequency (TF-IDF) Calculation

TF-IDF is widely used to evaluate the importance of a set of words in the fields of information retrieval and text mining. It is a combination of term frequency (TF) and inverse document frequency (IDF). The idea behind this combination is It evaluates the importance of a word within a document, and It evaluates the importance of the word among the collection of all documents

The TF-IDF formula has the following form:

\[TF-IDF_{i,j} = TF_{i,j} \times IDF_{i}\]

where i stands for the [math]\displaystyle{ i^{th} }[/math] word and j stands for the [math]\displaystyle{ j^{th} }[/math] document.

Term Frequency (TF)

TF evaluates the percentage of a given word in a document. Thus, TF value indicates the importance of a word. The TF has a positive relation with the importance.

In this paper, we only calculate TF for words in the keyword dictionary obtained. For a given keyword i, [math]\displaystyle{ TF_{i,j} }[/math] is the number of times word i appears in document j divided by the total number of words in document j.

The formula for TF has the following form:

\[TF_{i,j} = \frac{n_{i,j} }{\sum_k n_{k,j} }\]

where i stands for the [math]\displaystyle{ i^{th} }[/math] word, j stands for the [math]\displaystyle{ j^{th} }[/math] document, and [math]\displaystyle{ n_{i,j} }[/math] stands for the number of times words i appear in document j.

Note that the denominator is the total number of words remaining in document j after crawling.

Document Frequency (DF)

DF evaluates the percentage of documents that contain a given word over the entire collection of documents. Thus, the higher DF value is, the less important the word is. Since DF and the importance of the word have an inverse relation, we use IDF instead of DF.

[math]\displaystyle{ DF_{i} }[/math] is the number of documents in the collection with word i divided by the total number of documents in the collection. The formula for DF has the following form:

\[DF_{i} = \frac{|d_k \in D: n_{i,k} > 0|}{|D|}\]

where [math]\displaystyle{ n_{i,k} }[/math] is the number of times word i appears in document k, |D| is the total number of documents in the collection.

Inverse Document Frequency (IDF)

In this paper, IDF is calculated in a log scale. Since we will receive a large number of documents, i.e, we will have a large |D|

The formula for IDF has the following form:

\[IDF_{i} = log(\frac{|D|}{|d_k \in D: n_{i,k} > 0|})\]

As mentioned before, we will use HDFS. The actual formula applied is:

\[IDF_{i} = log(\frac{|D|+1}{|d_k \in D: n_{i,k} > 0|+1})\]

Paper Classification Using K-means Clustering

The K-means clustering is an unsupervised classification algorithm that groups similar data into the same class. It is an efficient and simple method that can work with different types of data attributes and is able to handle noise and outliers.

Given a set of [math]\displaystyle{ d \times n }[/math] dataset [math]\displaystyle{ \mathbf{X} = \left[ \mathbf{x}_1 \cdots \mathbf{x}_n \right] }[/math], the algorithm will assign each [math]\displaystyle{ \mathbf{x}_j }[/math] into [math]\displaystyle{ k }[/math] different clusters based on the characteristics of [math]\displaystyle{ \mathbf{x}_j }[/math] itself.

Moreover, when assigning data into a cluster, the algorithm will also try to minimise the distances between the data and the centre of the cluster which the data belongs to. That is, k-means clustering will minimise the sum of square error:

[math]\displaystyle{ min \sum_{i=1}^{k} \sum_{j \in C_i} ||x_j - \mu_i||^2 }[/math]

where

• [math]\displaystyle{ k }[/math]: the number of clusters
• [math]\displaystyle{ C_i }[/math]: the [math]\displaystyle{ i^th }[/math] cluster
• [math]\displaystyle{ x_j }[/math]: the [math]\displaystyle{ j^th }[/math] data in the [math]\displaystyle{ C_i }[/math]
• [math]\displaystyle{ mu_i }[/math]: the centroid of [math]\displaystyle{ C_i }[/math]
• [math]\displaystyle{ ||x_j - \mu_i||^2 }[/math]: the Euclidean distance between [math]\displaystyle{ x_j }[/math] and [math]\displaystyle{ \mu_i }[/math]

Since the goal for this paper is to classify research papers and group papers with similar topics based on keywords, the paper uses the K-means clustering algorithm. The algorithm first computes the cluster centroid for each group of papers with a specific topic. Then, it will assign a paper into a cluster based on the Euclidean distance between the cluster centroid and the paper’s TF-IDF value.

However, different values of [math]\displaystyle{ k }[/math] (the number of clusters) will return different clustering results. Therefore, it is important to define the number of clusters before clustering. For example, in this paper, the authors choose to use the Elbow scheme to determine the value of [math]\displaystyle{ k }[/math]. Also, to measure the performance of clustering, the authors decide to use the Silhouette scheme. The results of clustering are validated if the Silhouette scheme returns a value greater than [math]\displaystyle{ 0.5 }[/math].