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Summary by Shagun Sodhani 2 years ago
#### Introduction
* Introduces a new global log-bilinear regression model which combines the benefits of both global matrix factorization and local context window methods.
#### Global Matrix Factorization Methods
* Decompose large matrices into low-rank approximations.
* eg - Latent Semantic Analysis (LSA)
##### Limitations
* Poor performance on word analogy task
* Frequent words contribute disproportionately high to the similarity measure.
#### Shallow, Local Context-Based Window Methods
* Learn word representations using adjacent words.
* eg - Continous bag-of-words (CBOW) model and skip-gram model.
##### Limitations
* Since they do not operate directly on the global co-occurrence counts, they can not utilise the statistics of the corpus effectively.
#### GloVe Model
* To capture the relationship between words $i$ and $j$, word vector models should use ratios of co-occurene probabilites (with other words $k$) instead of using raw probabilites themselves.
* In most general form:
* $F(w_{i}, w_{j}, w_{k}^{~} ) = P_{ik}/P_{jk}$
* We want $F$ to encode information in the vector space (which have a linear structure), so we can restrict to the difference of $w_{i}$ and $w_{j}$
* $F(w_{i} - w_{j}, w_{k}^{~} ) = P_{ik}/P_{jk}$
* Since right hand side is a scalar and left hand side is a vector, we take dot product of the arguments.
* $F( (w_{i} - w_{j})^{T}, w_{k}^{~} ) = P_{ik}/P_{jk}$
* *F* should be invariant to order of the word pair $i$ and $j$.
* $F(w_{i}^{T}w_{k}^{~}) = P_{ik}$
* Doing further simplifications and optimisations (refer paper), we get cost function,
* $J = \sum_{\text{over all i, j pairs in the vocabulary}}[w_{i}^{T}w_{k}^{~} + b_{i} + b_{k}^{~} - log(X_{ik})]^{2}$
* $f$ is a weighing function.
* $f(x) = min((x/x_{max})^{\alpha}, 1)$
* Typical values, $x_{\max} = 100$ and $\alpha = 3/4$
* *b* are the bias terms.
##### Complexity
* Depends on a number of non-zero elements in the input matrix.
* Upper bound by the square of vocabulary size
* Since for shallow window-based approaches, complexity depends on $|C|$ (size of the corpus), tighter bounds are needed.
* By modelling number of co-occurrences of words as power law function of frequency rank, the complexity can be shown to be proportional to $|C|^{0.8}$
#### Evaluation
##### Tasks
* Word Analogies
* a is to b as c is to ___?
* Both semantic and syntactic pairs
* Find closest d to $w_{b} - w_{c} + w_{a}$ (using cosine similarity)
* Word Similarity
* Named Entity Recognition
##### Datasets
* Wikipedia Dumps - 2010 and 2014
* Gigaword5
* Combination of Gigaword5 and Wikipedia2014
* CommonCrawl
* 400,000 most frequent words considered from the corpus.
##### Hyperparameters
* Size of context window.
* Whether to distinguish left context from right context.
* $f$ - Word pairs that are $d$ words apart contribute $1/d$ to the total count.
* $xmax = 100$
* $\alpha = 3/4$
* AdaGrad update
##### Models Compared With
* Singular Value Decomposition
* Continous Bag-Of-Words
* Skip-Gram
##### Results
* Glove outperforms all other models significantly.
* Diminishing returns for vectors larger than 200 dimensions.
* Small and asymmetric context windows (context window only to the left) works better for syntactic tasks.
* Long and symmetric context windows (context window to both the sides) works better for semantic tasks.
* Syntactic task benefited from larger corpus though semantic task performed better with Wikipedia instead of Gigaword5 probably due to the comprehensiveness of Wikipedia and slightly outdated nature of Gigaword5.
* Word2vec’s performance decreases if the number of negative samples increases beyond about 10.
* For the same corpus, vocabulary, and window size GloVe consistently achieves better results, faster.

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