Paragraph Vectors has been recently proposed as an unsupervised method for learning distributed representations for pieces of texts. In their work, the authors showed that the method can learn an embedding of movie review texts which can be leveraged for sentiment analysis. That proof of concept, while encouraging, was rather narrow. Here we consider tasks other than sentiment analysis, provide a more thorough comparison of Paragraph Vectors to other document modelling algorithms such as Latent Dirichlet Allocation, and evaluate performance of the method as we vary the dimensionality of the learned representation. We benchmarked the models on two document similarity data sets, one from Wikipedia, one from arXiv. We observe that the Paragraph Vector method performs significantly better than other methods, and propose a simple improvement to enhance embedding quality. Somewhat surprisingly, we also show that much like word embeddings, vector operations on Paragraph Vectors can perform useful semantic results.READ FULL TEXT VIEW PDF
Central to many language understanding problems is the question of knowledge representation: How to capture the essential meaning of a document in a machine-understandable format (or “representation”). Despite much work going on in this area, the most established format is perhaps the bag of words (or bag of n-gram) representations. Latent Dirichlet Allocation (LDA)  is another widely adopted representation.
A recent paradigm in machine intelligence is to use a distributed representation for words  and documents . The interesting part is that even though these representations are less human-interpretable than previous representations, they seem to work well in practice. In particular, Le and Mikolov 
show that their method, Paragraph Vectors, capture many document semantics in dense vectors and that they can be used in classifying movie reviews or retrieving web pages.
Despite their success, little is known about how well the model works compared to Bag-of-Words or LDA for other unsupervised applications and how sensitive the model is when we change the hyperparameters.
In this paper, we make an attempt to compare Paragraph Vectors with other baselines in two tasks that have significant practical implications. First, we benchmark Paragraph Vectors on the task of Wikipedia browsing: given a Wikipedia article, what are the nearest articles that the audience should browse next. We also test Paragraph Vectors on the task of finding related articles on arXiv. In both of these tasks, we find that Paragraph Vectors allow for finding documents of interest via simple and intuitive vector operations. For example, we can find the Japanese equivalence of “Lady Gaga.”.
The goal of the paper is beyond benchmarking: The positive results on Wikipedia and arXiv datasets confirm that having good representations for texts can be powerful when it comes to language understanding. The success in these tasks shows that it is possible to use Paragraph Vectors for local and non-local browsing of large corpora.
We also show a simple yet effective trick to improve Paragraph Vector. In particular, we observe that by jointly training word embeddings, as in the skip gram model, the quality of the paragraph vectors is improved.
The Paragraph Vector model is first proposed in . The model inserts a memory vector to the standard language model which aims at capturing the topics of the document. The authors named this model “Distributed Memory”:
As suggested by the figure above, the paragraph vector is concatenated or averaged with local context word vectors to predict the next word. The prediction task changes the word vectors and the paragraph vector.
The paragraph vector can be further simplified when we use no local context in the prediction task. We can arrive at the following “Distributed Bag of Words” model:
At inference time, the parameters of the classifier and the word vectors are not needed and backpropagation is used to tune the paragraph vectors.
As the distributed bag of words model is more efficient, the experiments in this paper focuses on this implementation of the Paragraph Vector. In the following sections, we will explore the use of Paragraph Vectors in different applications in document understanding.
We conducted experiments with two different publicly available corpora: a corpus from the repository of electronic preprints (arXiv), and a corpus from the online encyclopaedia (Wikipedia).
In each case, all words were lower-cased before the datasets were used. We also jointly trained word embeddings with the paragraph vectors since preliminary experiments showed that this can improve the quality of the paragraph vectors. Preliminary results also showed that training with both unigrams and bigrams does not improve the quality of the final vectors. We present a range of qualitative and quantitative results. We give some examples of nearest neighbours to some Wikipedia articles and arXiv papers as well as a visualisation of the space of Wikipedia articles. We also show some examples of nearest neighbours after performing vector operations.
For the quantitative evaluation, we attempt to measure how well paragraph vectors represent semantic similarity of related articles. We do this by constructing (both automatically and by hand) triplets, where each triplet consists of a pair of items that are close to each other and one item that is unrelated.
For the publicly available corpora we trained paragraph vectors over at least 10 epochs of the data and use a hierarchical softmax constructed as a Huffman tree as the classifier. We use cosine similarity as the metric. We also applied LDA with Gibbs sampling and 500 iterations with varying numbers of topics. For LDA, we setto 0.1 and used values of between 0.01 and 0.000001. We used the posterior topic proportions for each paper with Hellinger distance to compute the similarity between pairs of documents. For completeness, we also include the results of averaging the word embeddings for each word in a paper and using that as the paragraph vector. Finally, we consider the classical bag of words model where each word is represented as a one-hot vector weighted by TF-IDF and the document is represented by that vector, with comparisons done using cosine similarity.
We extracted the main body text of 4,490,000 Wikipedia articles from the English site. We removed all links and applied a frequency cutoff to obtain a vocabulary of 915,715 words. We trained paragraph vectors on these Wikipedia articles and visualized them in Figure 3 using t-SNE . The visualization confirms that articles having the same category are grouped together. There is a wide range of sport descriptions on wikipedia, which explains why the sports are less concentrated.
We also qualitatively look at the nearest neighbours of Wikipedia articles and compare Paragraph Vectors and LDA. For example, the nearest neighbours for the Wikipedia article “Machine learning” are shown in Table1. We find that overall Paragraph Vectors have better nearest neighbours than LDA.
Artificial neural network
|Artificial neural network|
|Predictive analytics||Types of artificial neural networks|
|Structured prediction||Unsupervised learning|
|Mathematical geophysics||Feature learning|
|Supervised learning||Predictive analytics|
|Constrained conditional model||Pattern recognition|
|Sensitivity analysis||Statistical classification|
|Feature scaling||Training set|
|Boosting (machine learning)||Meta learning (computer science)|
|Prior probability||Kernel method|
|Curse of dimensionality||Supervised learning|
|Scientific evidence||Generalization error|
|Online machine learning||Overfitting|
|Cluster analysis||Generative model|
|Dimensionality reduction||Computational learning theory|
|Functional decomposition||Inductive bias|
|Bayesian network||Semi-supervised learning|
We can perform vector operations on paragraph vectors for local and non-local browsing of Wikipedia. In Table (a)a and Table (b)b, we show results of two experiments. The first experiment is to find related articles to “Lady Gaga.” The second experiment is to find the Japanese equivalence of “Lady Gaga.” This can be achieved by vector operations: (“Lady Gaga”) - (“American”) + (“Japanese”) where is paragraph vectors and is word vectors. Both sets of results show that Paragraph Vectors can achieve the same kind of analogies like Word Vectors .
To quantitatively compare these methods, we constructed two datasets for triplet evaluation. The first consists of 172 triplets of articles we knew were related because of our domain knowledge. Some examples are: “Deep learning” is closer to “Machine learning” than “Computer network” or “Google” is closer to “Facebook” than “Walmart” etc. Some examples are hard and probably require some deep understanding of the content such as “San Diego” is closer to “Los Angeles” than “San Jose.”
The second dataset consists of 19,876 triplets in which two articles are closer because they are listed in the same category by Wikipedia, and the last article is not in the same category, but may be in a sibling category. For example, the articles for “Barack Obama” are closer to “Britney Spears” than “China.” These triplets are generated randomly.111The datasets are available at http://cs.stanford.edu/~quocle/triplets-data.tar.gz
We will benchmark document embedding methods, such as LDA, bag of words, Paragraph Vector, to see how well these models capture the semantic of the documents. The results are reported in Table 3 and Table 4. For each of the methods, we also vary the number of embedding dimensions.
|Averaged word embeddings||3000||84.9%|
|Bag of words||86.0%|
|Averaged word embeddings||3000||74%|
|Bag of words||78.3%|
From the results in Table 3 and 4, it can be seen that paragraph vectors perform better than LDA. We also see a peak in paragraph vector performance at 10,000 dimensions. Both paragraph vectors and averaging word embeddings perform better than the LDA model. For LDA, we found that TF-IDF weighting of words and their inferred topic allocations did not affect the performance. From these results, we can also see that joint training of word vectors improves the final quality of the paragraph vectors.
We extracted text from the PDF versions of 886,000 full arXiv papers. In each case we only used the latest revision available. We applied a minimum frequency cutoff to the vocabulary so that our final vocabulary was 969,894 words.
We performed experiments to find related articles using Paragraph Vectors. In Table 5 and Table 6, we show the nearest neighbours of the original Paragraph Vector paper “Distributed Representations of Sentences and Documents” and the current paper. In Table 7, we want to find the Bayesian equivalence of the Paragraph Vector paper. This can be achieved by vector operations: (“Distributed Representations of Sentences and Documents”) - (“neural”) + (“Bayesian”) where are paragraph vectors and are word vectors learned during the training of paragraph vectors. The results suggest that Paragraph Vector works well in these two tasks.
Evaluating Neural Word Representations in Tensor-Based Compositional Settings
|Polyglot: Distributed Word Representations for Multilingual NLP||0.764|
|Lexicon Infused Phrase Embeddings for Named Entity Resolution||0.757|
A Convolutional Neural Network for Modelling Sentences
|Distributed Representations of Words and Phrases and their Compositionality||0.740|
|Convolutional Neural Networks for Sentence Classification||0.735|
SimLex-999: Evaluating Semantic Models With (Genuine) Similarity Estimation
|Exploiting Similarities among Languages for Machine Translation||0.731|
|Efficient Estimation of Word Representations in Vector Space||0.727|
|Multilingual Distributed Representations without Word Alignment||0.721|
|Distributed Representations of Sentences and Documents||0.681|
|Efficient Estimation of Word Representations in Vector Space||0.680|
|Thumbs up? Sentiment Classification using Machine Learning Techniques||0.642|
|Distributed Representations of Words and Phrases and their Compositionality||0.624|
|KNET: A General Framework for Learning Word Embedding using||0.622|
|Japanese-Spanish Thesaurus Construction Using English as a Pivot||0.614|
|Multilingual Distributed Representations without Word Alignment||0.614|
|Catching the Drift: Probabilistic Content Models, with Applications||0.613|
|to Generation and Summarization|
|Exploiting Similarities among Languages for Machine Translation||0.612|
|A Survey on Information Retrieval, Text Categorization, and Web Crawling||0.610|
|Content Modeling Using Latent Permutations||0.629|
|SimLex-999: Evaluating Semantic Models With (Genuine) Similarity Estimation||0.611|
|Probabilistic Topic and Syntax Modeling with Part-of-Speech LDA||0.579|
|Evaluating Neural Word Representations in Tensor-Based Compositional Settings||0.572|
|Syntactic Topic Models||0.548|
Training Restricted Boltzmann Machines on Word Observations
|Discrete Component Analysis||0.547|
|Resolving Lexical Ambiguity in Tensor Regression Models of Meaning||0.546|
|Measuring political sentiment on Twitter: factor-optimal design for||0.544|
|multinomial inverse regression|
|Scalable Probabilistic Entity-Topic Modeling||0.541|
To measure the performance of different models on this task, we picked pairs of papers that had at least one shared subject, the unrelated paper was chosen at random from papers with no shared subjects with the first paper. We produced a dataset of 20,000 triplets by this method.
|Averaged word embeddings||300||81.1%|
|Bag of words||80.4%|
From the results in Table 8, it can be seen that paragraph vectors perform on par than the best performing number of topics for LDA. Paragraph Vectors are also less sensitive to differences in embedding size than LDA is to the number of topics. We also see a peak in paragraph vector performance at 100 dimensions. Both models perform better than the vector space model. For LDA, we found that TF-IDF weighting of words and their inferred topic allocations did not affect performance.
We described a new set of results on Paragraph Vectors showing they can effectively be used for measuring semantic similarity between long pieces of texts. Our experiments show that Paragraph Vectors are superior to LDA for measuring semantic similarity on Wikipedia articles across all sizes of Paragraph Vectors. Paragraph Vectors also perform on par with LDA’s best performing number of topics on arXiv papers and perform consistently relative to the embedding size. Also surprisingly, vector operations can be performed on them similarly to word vectors. This can provide interesting new techniques for a wide range of applications: local and nonlocal corpus navigation, dataset exploration, book recommendation and reviewer allocation.
Visualizing high-dimensional data using t-SNE.Journal of Machine Learning Research, 2008.