| pre | keyword | post |
|---|---|---|
| can be delivered for the | banking | industry in Europe . I |
| instance I am referring to | banking | . It is not only |
| that , if the second | banking | directive comes into force without |
| the future of the British | banking | industry within the European Community |
| the future of the British | banking | industry within the European Community |
| the Government expect the second | banking | directive to come into force |
7: Word Embeddings
Jack Blumenau
Module feedback
Module feedback
Module feedback
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Module feedback
Could we also go over the application of ‘content’ argument for the topic model (in stm function)?
Can we have the code that produce the graph you showed us in the lecture sildes?
Please give us more guidance in cleaning text related data to better conduct analysis.
Response: I will add materials on these questions to the course website.
Module feedback
have difficulty in coming up with research questions and get access to appropriate data sources
Please could you release examples of good research projects from students from previous years?
Response: I will add some examples to the course website. If you are struggling, please come to see me during office hours.
Module feedback
The slides are a bit dense, it can make it difficult to focus on the content when you’re fighting to get everything written down.
We go through content quite quickly, and it’s hard to get all the seminar work done in class, but I’m liking the course!
The lecture slides are very long and hard to get through. We understand there’s so much to get through but sometimes it’s too much The seminar is amazing
Response: I will try to slim things down and to go slower
Introduction to Word Embeddings
Sparse Representations of Words
Up until this point of the course, we have always implicitly used representations of words that are sparse.
Words were represented as one-hot encoding, i.e. word-specific vectors that take the value of 1 only for that word, and 0 for all others. E.g.
\[\begin{align} w_{\text{debt}} &= \begin{bmatrix} 0, & 0, & 1, & 0, & ..., & 0 \end{bmatrix} \end{align}\]
\[\begin{align} w_{\text{deficit}} &= \begin{bmatrix} 0, & 0, & 0, & 1, & ..., & 0 \end{bmatrix} \end{align}\]
The problem with this representation is that they contain no notion of similarity between words.
The dot product between two word vectors is zero:
\[ cos(\theta) = w_{\text{deficit}}^Tw_{\text{debt}} = \frac{\mathbf{w_{\text{deficit}}} \cdot \mathbf{w_{\text{debt}}}}{\left|\left| \mathbf{w_{\text{deficit}}} \right|\right| \left|\left| \mathbf{w_{\text{debt}}} \right|\right|}= 0 \]
This is true for any pair of words, which is clearly nonsense as some pairs of words are more similar to each other than other pairs of words.
Problems with Sparse Word Representations
Mechanical problems:
Similarity
- Documents might have zero term overlap, but have nearly identical meanings
- E.g. “Quantitative text analysis is very successful.” vs “Natural language processing is tremendously effective.”
Classification/Dictionaries/Supervised scaling
- We may know or learn that one word is connected to a concept, but that doesn’t tell us anything about other similar words
- If we learn that “turmeric” is highly predictive of the concept of interest, shouldn’t we also learn something about “garlic”, “saffron”, and “ginger”?
Topic models/Unsupervised scaling
- If “bank”, “economy”, “interest”, and “rates” have high probability under a topic, shouldn’t “monetary” also have high probability?
Substantive problem: Our representation does not encode any information about the “meaning” of a word. The binary representation implies that all words are distinct to an equal degree, which seems a strong assumption.
\(\Rightarrow\) We would prefer a representation that allows us to capture similarities between word meanings.
Distributional Semantics
The distributional hypothesis: the meaning of a word can be derived from the distribution of contexts in which it appears.
We can learn about the meaning of a word by investigating the distribution of words that show up around the word
- “You shall know a word by the company it keeps!” J.R. Firth (1957)
- “The meaning of words lies in their use” Ludwig Wittgenstein (1953)
The hypothesis implies that words that appear in similar “contexts” will share similar meanings
This simple (and old) idea is one of the most influential and successful ideas in modern natural language processing
Word embedding approaches represent the distributional “meaning” of a word as a vector in multidimensional space
Distributional Semantics
When a word \(j\) appears in a text, its “context” is the set of words that appear nearby (within a fixed-size window)
We use the many contexts of \(w\) to build up a representation of \(w\)
| pre | keyword | post |
|---|---|---|
| during the passage of the | Finance | Bill , but I can |
| is referring to taxpayers ' | finance | and public sector funding , |
| world when it comes to | finance | . The industry is being |
| of the European Council of | Finance | Ministers with his Community colleagues |
| of the European Council of | Finance | Ministers with his Community colleagues |
| practically involved in local government | finance | . It must have come |
Word Embedding Overview
The meaning of each word is based on the distribution of terms with which it co-occurs
We represent this meaning using a vector for each word
Vectors are constructed such that similar words are close to each other in “semantic” space
We build this space automatically by seeing which words are close to one another in texts
Dense Representations of Words
Our goal will be to build a dense vector for each word, chosen so that it is similar to vectors of words that appear in similar contexts (measuring similarity as the dot product)
\[\begin{align} w_{\text{debt}} &= \begin{bmatrix} 0.73 \\ 0.04 \\ 0.07 \\ -0.18 \\ 0.81 \\ -0.97 \end{bmatrix} \end{align}\]
\[\begin{align} w_{\text{deficit}} &= \begin{bmatrix} 0.63 \\ .14 \\ .02 \\ -0.58 \\ 0.43 \\ -0.66 \end{bmatrix} \end{align}\]
These representations are known as word embeddings because we “embed” words into a low-dimensional space (low compared to the vocabulary size).
Advantages of Word Embeddings
Low-dimensional word embeddings offer three core advantages over simple word counts. They:
Encode similarity between words
- We no longer have word similarities of zero!
- Each word is a vector, with the vectors of similar words closer together than vectors of very different words
Allow for “automatic generalization”
- Imagine that we discover “fantastic” is a good predictor of positive reviews, but we never observe the word “extraordinary” in our training corpus
- Because “fantastic” and “extraordinary” will have similar word vectors, we can share information across words, and apply what we have learned about one word to our understanding of another
- Can lead to large performance gains for prediction and topic modelling tasks
Provide a measure of meaning
- We will often be interested in the meaning of words as a quantity in its own right
- Very specific meaning of “meaning”: two words share a meaning when used in similar contexts
Contrasting Approaches
The material that we study this week is different on several dimensions from the approaches on the course so far:
| Traditional Unsupervised | Traditional Supervised | Word Embeddings | |
|---|---|---|---|
| Bag of words? | Yes | Yes | No |
| Inputs | DFM | DFM | Feature co-occurence matrix |
| Outputs | Topics;Scales | Categorization | Word vectors |
Estimating Word Embeddings
Design Choices in Word Embeddings
Data
- High-quality embeddings require a large amount of training data
- Usually trained on large external corpora (i.e. wikipedia; news articles; web pages)
- Embeddings will reflect the language used in the training documents
Context Window Size
- If meaning is defined by a word’s context, we need to define context
- Usually implemented as a symmetric window of some length around each word
- The size of the window dictates the kind of information the embedding will capture
Dimension of the Embedding
- The embedding for each word will typically be between 50 and 500 elements long
- The embedding encodes information about the contexts a word appears in, so in theory larger embeddings are able to encode more information
- In practice, medium-dimensional embeddings (100-300) work very well
Algorithm
- There are several algorithms that allow us to learn embeddings from data
- Count-based, sparse approaches (e.g. co-occurence vectors)
- Neural network, dense approaches (e.g “Word2Vec” and “GloVe”)
Co-occurence Vectors
One simple “embedding” is produced by counting the occurrences of any term within a fixed window of any other term and store these in a feature-cooccurence matrix:
Feature co-occurrence matrix of: 327,761 by 327,761 features.
features
features before the house proceeds to choice of a speaker ,
before 948 119351 17939 89 28688 213 21405 18847 277 47978
the 119351 3792520 580515 4167 3629935 13840 6681402 628635 18558 3962528
house 17939 580515 2006 112 150334 176 211732 33340 1401 124961
proceeds 89 4167 112 6 817 1 2572 197 0 870
to 28688 3629935 150334 817 1053898 7231 736710 957469 5363 1291597
choice 213 13840 176 1 7231 298 9407 10100 31 8430
of 21405 6681402 211732 2572 736710 9407 755200 1244591 2787 1491028
a 18847 628635 33340 197 957469 10100 1244591 161468 2751 985450
speaker 277 18558 1401 0 5363 31 2787 2751 412 83721
, 47978 3962528 124961 870 1291597 8430 1491028 985450 83721 1766522
[ reached max_feat ... 327,751 more features, reached max_nfeat ... 327,751 more features ]The word “house” appears within three words of the word “before” 17939 times in this corpus
The word “speaker” never appears within three words of “proceeds” in this corpus
Etc
Typically, these vectors are then weighted by the pointwise mutual information:
\[PMI(w,c) = log\frac{P(w,c)}{P(w)P(c)}\]
\(\rightarrow\) PMI compares the probability of two words occurring together to what this probability would be if the words were unrelated (independent in probabilities)
\(\rightarrow\) larger weight on feature combinations that are more frequent, lower weights on features that commonly occur with many other features (similar intuition to tf-idf).
Co-occurence Vectors
Given this representation, we can calculate the cosine similarity between the word vectors of target words to find the closest other words in the embedding space:
election elections referendum general electoral elected manifesto party vote labour
1.0000000 0.2697683 0.2190709 0.2078489 0.1907036 0.1864098 0.1858062 0.1847441 0.1841158 0.1833788 health mental nhs care service services social healthcare education medical
1.0000000 0.3688996 0.2943339 0.2893435 0.2541648 0.2453370 0.2348707 0.2285000 0.2264886 0.2239171 banking banks lending financial bank institutions corporate consumer regulatory markets
1.0000000 0.2769608 0.2318190 0.2301063 0.2215138 0.2132605 0.2086125 0.2057390 0.2057376 0.2046487 Co-occurence Vectors
Co-occurence vectors clearly capture something about the meaning of words
However, these vectors increase in the size of the vocabulary of the corpus
- The vectors above each have length 327761
One consequence is that they tend to be very sparse (most words fail to occur with most other words)
- E.g. the sparsity of the example above is 99.9%
In most applications, sparse vectors like these tend to perform less well than dense vectors
- Similarity; classification; unsupervised learning, etc
We would therefore prefer a low-dimensional representation that didn’t suffer from these sparsity issues
Word-2-Vec Overview
Word2Vec (Mikolov et al, 2013) is a set of related methods for learning dense word vectors.
One version of Word2Vec – skip-gram with negative sampling – follows this basic process:
Start with a very large corpus of text (i.e. all of Wikipedia)
Represent each word in the vocabulary as a vector, \(\mu_j\)
- Initialise each vector with random numbers
Go through each position, \(t\), in the text, where each position has
- A center word, \(t\) (“target” words)
- Context words, \(o\) (“outside” words)
Calculate the probability of observing \(o\) given \(t\) (or vice versa), using the similarity of the word vectors for \(o\) and \(t\)
Adjust the values of the word vectors, \(\mu_j\), to …
- …maximize the probability of observing true context words
- …minimize the probability of observing other words from the corpus
Word-2-Vec Intuition
What is the probability of observing the context words given the center word, ‘UN’?
\[\begin{equation} \text{important }\underbrace{\text{\textcolor{orange}{to get }}}_{\substack{\text{Context words}\\ \text{window of size 2}}}\underbrace{\text{\textcolor{red}{UN }}}_{\substack{\text{Center word at }\\ \text{position $t$}}}\underbrace{\text{\textcolor{orange}{agreement as }}}_{\substack{\text{Context words}\\ \text{window of size 2}}}\text{the last hope of demonstrating international agreement }\end{equation}\]\(p(w_o = \text{to}|w_t = \text{UN})\)
\(p(w_o = \text{get}|w_t = \text{UN})\)
\(p(w_o = \text{agreement}|w_t = \text{UN})\)
\(p(w_o = \text{as}|w_t = \text{UN})\)
What is the probability of observing the context words given the center word, ‘agreement’?
\[\begin{equation} \text{important to }\underbrace{\text{\textcolor{orange}{get UN }}}_{\substack{\text{Context words}\\ \text{window of size 2}}}\underbrace{\text{\textcolor{red}{agreement }}}_{\substack{\text{Center word at }\\ \text{position $t$}}}\underbrace{\text{\textcolor{orange}{as the }}}_{\substack{\text{Context words}\\ \text{window of size 2}}}\text{last hope of demonstrating international agreement }\end{equation}\]\(p(w_o = \text{get}|w_t = \text{agreement})\)
\(p(w_o = \text{UN}|w_t = \text{agreement})\)
\(p(w_o = \text{as}|w_t = \text{agreement})\)
\(p(w_o = \text{the}|w_t = \text{agreement})\)
What is the probability of observing the context words given the center word, ‘as’?
\[\begin{equation} \text{important to get }\underbrace{\text{\textcolor{orange}{UN agreement }}}_{\substack{\text{Context words}\\ \text{window of size 2}}}\underbrace{\text{\textcolor{red}{as }}}_{\substack{\text{Center word at }\\ \text{position $t$}}}\underbrace{\text{\textcolor{orange}{the last }}}_{\substack{\text{Context words}\\ \text{window of size 2}}}\text{hope of demonstrating international agreement }\end{equation}\]\(p(w_o = \text{UN}|w_t = \text{as})\)
\(p(w_o = \text{agreement}|w_t = \text{as})\)
\(p(w_o = \text{the}|w_t = \text{as})\)
\(p(w_o = \text{last}|w_t = \text{as})\)
What is the probability of observing the context words given the center word, ‘the’?
\[\begin{equation} \text{important to get UN }\underbrace{\text{\textcolor{orange}{agreement as }}}_{\substack{\text{Context words}\\ \text{window of size 2}}}\underbrace{\text{\textcolor{red}{the }}}_{\substack{\text{Center word at }\\ \text{position $t$}}}\underbrace{\text{\textcolor{orange}{last hope }}}_{\substack{\text{Context words}\\ \text{window of size 2}}}\text{of demonstrating international agreement }\end{equation}\]\(p(w_o = \text{agreement}|w_t = \text{the})\)
\(p(w_o = \text{as}|w_t = \text{the})\)
\(p(w_o = \text{last}|w_t = \text{the})\)
\(p(w_o = \text{hope}|w_t = \text{the})\)
What is the probability of observing the context words given the center word, ‘last’?
\[\begin{equation} \text{important to get UN agreement }\underbrace{\text{\textcolor{orange}{as the }}}_{\substack{\text{Context words}\\ \text{window of size 2}}}\underbrace{\text{\textcolor{red}{last }}}_{\substack{\text{Center word at }\\ \text{position $t$}}}\underbrace{\text{\textcolor{orange}{hope of }}}_{\substack{\text{Context words}\\ \text{window of size 2}}}\text{demonstrating international agreement }\end{equation}\]\(p(w_o = \text{as}|w_t = \text{last})\)
\(p(w_o = \text{the}|w_t = \text{last})\)
\(p(w_o = \text{hope}|w_t = \text{last})\)
\(p(w_o = \text{of}|w_t = \text{last})\)
What is the probability of observing the context words given the center word, ‘hope’?
\[\begin{equation} \text{important to get UN agreement as }\underbrace{\text{\textcolor{orange}{the last }}}_{\substack{\text{Context words}\\ \text{window of size 2}}}\underbrace{\text{\textcolor{red}{hope }}}_{\substack{\text{Center word at }\\ \text{position $t$}}}\underbrace{\text{\textcolor{orange}{of demonstrating }}}_{\substack{\text{Context words}\\ \text{window of size 2}}}\text{international agreement }\end{equation}\]\(p(w_o = \text{the}|w_t = \text{hope})\)
\(p(w_o = \text{last}|w_t = \text{hope})\)
\(p(w_o = \text{of}|w_t = \text{hope})\)
\(p(w_o = \text{demonstrating}|w_t = \text{hope})\)
Word2Vec Objective Function
- The objective of the Word2Vec model is to maximise the average log probability:
\[ \frac{1}{T}\sum_{t=1}^T \sum_{-c \leq j \leq c, j\neq0} \text{log} p(w_{t+j}|w_t) \]
where probability, \(p(w_{t+j}|w_t)\), is defined as:
\[\begin{equation} p(w_{t+j}|w_t) = \frac{\text{exp}(v_{o}^T \cdot v_{t})}{\sum_{w=1}^W\text{exp}(v_{o}^T \cdot v_{t})} \end{equation}\]
- This is an example of the softmax function, which maps arbitrary values to a probability distribution.
- Goal: Learn values of the word-embeddings, \(v_w\), that maximise the probability of observing the context words that we actually do observe.
- Approach: We learn the values of the word-embeddings by stochastic gradient descent – we gradually adjust the values of the embeddings to maximise the probabilities above
- Doing no more than this allows the algorithm to learn word vectors that capture word similarity and meaningful directions in a word space
Learning Skip-Gram Embeddings (Negative Sampling)
The denominator of the softmax function defined above is very computationally expensive to evaluate repeatedly and so Word2Vec recasts the problem as supervised learning problem.
- Select word from position \(t\) and select the “positive” words (\(Y=1\)) that fall in its context (i.e. \(\pm 2\) words)
- For each true context word, select \(K\) “negative” context words (\(Y=0\)) at random from the entire corpus
- Run a logistic regression, with the positive/negative variable as outcome, and the dot product between the words’ embeddings as predictor (\(v_{o}^T \cdot v_{t}\))
- Adjust the values of the embeddings to better distinguish between the positive and negative context words
| \(t\) | \(o\) | \(Y\) |
|---|---|---|
| UN | to | 1 |
| UN | get | 1 |
| UN | agreement | 1 |
| UN | as | 1 |
| \(t\) | \(o\) | \(Y\) |
|---|---|---|
| UN | to | 1 |
| UN | get | 1 |
| UN | agreement | 1 |
| UN | as | 1 |
| UN | castle | 0 |
| UN | when | 0 |
| UN | chair | 0 |
| UN | yoyo | 0 |
| UN | pancake | 0 |
| UN | whilst | 0 |
| UN | tremmor | 0 |
| UN | foot | 0 |
| \(t\) | \(o\) | \(Y\) | \(v_{o}^T \cdot v_{t}\) |
|---|---|---|---|
| UN | to | 1 | 0.172 |
| UN | get | 1 | 0.226 |
| UN | agreement | 1 | 0.619 |
| UN | as | 1 | 0.351 |
| UN | castle | 0 | 0.112 |
| UN | when | 0 | -0.559 |
| UN | chair | 0 | -0.323 |
| UN | yoyo | 0 | 0.371 |
| UN | pancake | 0 | 0.106 |
| UN | whilst | 0 | 0.021 |
| UN | tremmor | 0 | 0.052 |
| UN | foot | 0 | 0.105 |
| \(t\) | \(o\) | \(Y\) | \(v_{o}^T \cdot v_{t}\) |
|---|---|---|---|
| UN | to | 1 | 0.279 |
| UN | get | 1 | 0.465 |
| UN | agreement | 1 | 0.36 |
| UN | as | 1 | 0.164 |
| UN | castle | 0 | -0.174 |
| UN | when | 0 | -0.23 |
| UN | chair | 0 | 0.043 |
| UN | yoyo | 0 | -0.19 |
| UN | pancake | 0 | -0.049 |
| UN | whilst | 0 | -0.106 |
| UN | tremmor | 0 | -0.02 |
| UN | foot | 0 | -0.208 |
The goal of the learning algorithm is to maximise the similarity of the target and positive context vectors, and minimize the similarity between the target and negative context word vectors.
Word2Vec Intuition
Question: Why do the estimated word-embeddings encode information about word similarity?
The predicted probability of a context word is high when the dot product between the context word’s embedding and the target word’s embedding is high
\(p(w_{t+j}|w_t) = \frac{\text{exp}(v_{o}^T \cdot v_{t})}{\sum_{w=1}^W\text{exp}(v_{o}^T \cdot v_{t})}\)
\(v_{o}^T \cdot v_{t} = \sum_{i=1}^n v_{o,i}v_{t,i}\)
This encourages the model to find embedding vectors that are similar to one another for words that occur together frequently in the corpus
It also encourages the model to find embedding vectors that are similar to one another for words that appear in similar contexts in the corpus, even if they rarely appear together. E.g.
- “worldcom” and “scandal” appear frequently together
- “enron” and “scandal” appear frequently together
- But “worldcom” and “enron” appear infrequently together
- “worldcom” and “enron” will still need relatively proximate embeddings in order to predict the occurrence of “scandal” in both contexts
GloVe: Global Vectors for Word Representation
- The Glove algorithm builds directly on the idea of the co-occurence vectors that we discussed previously
- Weighted least squares model that learns dense vectors from the word-word co-occurence counts
- GloVe models the log of the number of times that each word appears in the context of each other word:
\[\min_\theta J(\theta) \ \ \ \text{where} \ \ \ J(\theta) = \sum_{i=1}^V\sum_{i,j=1}^Vf(X_{i,j}) (v_i^T \cdot v_j - \text{log}(X_{i,j}))^2\]
Where
- \(X\) is a word-word co-occurence matrix
- \(X_{i,j}\) is the number of times word \(j\) appears in the context of word \(i\)
Intuition:
- The GloVe model tries to make it so that the dot product between the word vectors for \(i\) and \(j\) are equal to the log of the co-occurrence between the words
- \(f(X_{i,j})\) is a weighting function that puts somewhat higher weights on more infrequent words (so that very common words do not dominate)
GloVe vs Word2Vec
- The core difference between the two models is that GloVe is a model for the global co-occurence counts, while Word2Vec is an “online” model which trains progressively on a moving window
The GloVe model has some advantages over Word2Vec
- Very fast
- Easily scales to very large corpora
- Good performance on small corpora
- However, across many practical applications, there is no clear evidence that one model outperforms the other (Rodriguez and Spirling, 2020)
In both models, the researcher has to make several decisions that can be consequential to the estimated word vectors
- Context-window size
- Embedding dimensionality
- Pre-trained versus local fit
Glove Embeddings
num [1:400000, 1:300] 0.0466 -0.2554 -0.1256 -0.0769 -0.2576 ...
- attr(*, "dimnames")=List of 2
..$ : chr [1:400000] "the" "," "." "of" ...
..$ : NULL [1] 0.0465600 0.2131800 -0.0074364 -0.4585400 -0.0356390 0.2364300 -0.2883600 0.2152100 -0.1348600 -1.6413000 -0.2609100 0.0324340 0.0566210 -0.0432960 -0.0216720 0.2247600 -0.0751290 -0.0670180 -0.1424700 0.0388250 -0.1895100 0.2997700 0.3930500 0.1788700 -0.1734300 -0.2117800
[27] 0.2361700 -0.0636810 -0.4231800 -0.1166100 0.0937540 0.1729600 -0.3307300 0.4911200 -0.6899500 -0.0924620 0.2474200 -0.1799100 0.0979080 0.0831180 0.1529900 -0.2727600 -0.0389340 0.5445300 0.5373700 0.2910500 -0.0073514 0.0478800 -0.4076000 -0.0267590 0.1791900 0.0109770
[53] -0.1096300 -0.2639500 0.0739900 0.2623600 -0.1508000 0.3462300 0.2575800 0.1197100 -0.0371350 -0.0715930 0.4389800 -0.0407640 0.0164250 -0.4464000 0.1719700 0.0462460 0.0586390 0.0414990 0.5394800 0.5249500 0.1136100 -0.0483150 -0.3638500 0.1870400 0.0927610 -0.1112900
[79] -0.4208500 0.1399200 -0.3933800 -0.0679450 0.1218800 0.1670700 0.0751690 -0.0155290 -0.1949900 0.1963800 0.0531940 0.2517000 -0.3484500 -0.1063800 -0.3469200 -0.1902400 -0.2004000 0.1215400 -0.2920800 0.0233530 -0.1161800 -0.3576800 0.0623040 0.3588400 0.0290600 0.0073005
[105] 0.0049482 -0.1504800 -0.1231300 0.1933700 0.1217300 0.4450300 0.2514700 0.1078100 -0.1771600 0.0386910 0.0815300 0.1466700 0.0636660 0.0613320 -0.0755690 -0.3772400 0.0158500 -0.3034200 0.2837400 -0.0420130 -0.0407150 -0.1526900 0.0749800 0.1557700 0.1043300 0.3139300
[131] 0.1930900 0.1942900 0.1518500 -0.1019200 -0.0187850 0.2079100 0.1336600 0.1903800 -0.2555800 0.3040000 -0.0189600 0.2014700 -0.4211000 -0.0075156 -0.2797700 -0.1931400 0.0462040 0.1997100 -0.3020700 0.2573500 0.6810700 -0.1940900 0.2398400 0.2249300 0.6522400 -0.1356100
[157] -0.1738300 -0.0482090 -0.1186000 0.0021588 -0.0195250 0.1194800 0.1934600 -0.4082000 -0.0829660 0.1662600 -0.1060100 0.3586100 0.1692200 0.0725900 -0.2480300 -0.1002400 -0.5249100 -0.1774500 -0.3664700 0.2618000 -0.0120770 0.0831900 -0.2152800 0.4104500 0.2913600 0.3086900
[183] 0.0788640 0.3220700 -0.0410230 -0.1097000 -0.0920410 -0.1233900 -0.1641600 0.3538200 -0.0827740 0.3317100 -0.2473800 -0.0489280 0.1574600 0.1898800 -0.0266420 0.0633150 -0.0106730 0.3408900 1.4106000 0.1341700 0.2819100 -0.2594000 0.0552670 -0.0524250 -0.2578900 0.0191270
[209] -0.0220840 0.3211300 0.0688180 0.5120700 0.1647800 -0.2019400 0.2923200 0.0985750 0.0131450 -0.1065200 0.1351000 -0.0453320 0.2069700 -0.4842500 -0.4470600 0.0033305 0.0029264 -0.1097500 -0.2332500 0.2244200 -0.1050300 0.1233900 0.1097800 0.0489940 -0.2515700 0.4031900
[235] 0.3531800 0.1865100 -0.0236220 -0.1273400 0.1147500 0.2735900 -0.2186600 0.0157940 0.8175400 -0.0237920 -0.8546900 -0.1620300 0.1807600 0.0280140 -0.1434000 0.0013139 -0.0917350 -0.0897040 0.1110500 -0.1670300 0.0683770 -0.0873880 -0.0397890 0.0141840 0.2118700 0.2857900
[261] -0.2879700 -0.0589960 -0.0324360 -0.0047009 -0.1705200 -0.0347410 -0.1148900 0.0750930 0.0995260 0.0481830 -0.0737750 -0.4181700 0.0041268 0.4441400 -0.1606200 0.1429400 -2.2628000 -0.0273470 0.8131100 0.7741700 -0.2563900 -0.1157600 -0.1198200 -0.2136300 0.0284290 0.2726100
[287] 0.0310260 0.0967820 0.0067769 0.1408200 -0.0130640 -0.2968600 -0.0799130 0.1950000 0.0315490 0.2850600 -0.0874610 0.0090611 -0.2098900 0.0539130Context-window Size
The size of the context window determines which type of word meaning is represented in the embedding space
Small context windows (\(\pm\) 1-3 words) \(\rightarrow\) syntactic meaning
- E.g. putting, bringing, taking, giving, providing, etc
Medium context windows (\(\pm\) 5-10 words) \(\rightarrow\) semantic meaning
- E.g. crimes, crime, offences, offence, prosecutions, murder, etc
Large context windows (\(\pm\) 10+ words) \(\rightarrow\) topical meaning
- E.g. tourism, visitors, museum, holiday, cafe, etc
\(\Rightarrow\) the size of the window will depend on the research question.
Embedding Dimensions
The size of the embedding vectors determines how complex the model is that we wish to fit
We have an embedding for each word, so increasing the embedding dimension by 1 multiplies the number of parameters to estimate by \(V\) (the total number of words in the vocabulary)
Too many dimensions: higher chance of modelling noise
Too few dimensions: higher chance of missing important subtleties in meaning
General guidance: about 150-300 is fine (though this is not a very satisfying answer)
What do word-embedding dimensions “mean”?
- We can now generate multidimensional vectors for each of our words (we will see shortly) are very successful in capturing semantic relations among words
- This implies that a meaningful semantic structure must be present in the respective vector spaces
- However, it is very difficult to answer questions such as “what do high and low values of the \(i\)th embedding dimension mean?”
- Example:
[1] "samiul" "stuffit" "guangwei" "decompress" "resend" "sife" [1] "cheesecloth" "globe.com" "zubaie" "metrohealth" "25-march" "30-aug" [1] "republish" "12,000-page" "transmittable" "affray" "6-pica" "spongiform" - Generating “interpretable” word embeddings is the subject of ongoing work
Local Versus Pre-Trained
Either the Word2Vec or Glove methods can be applied to any large corpus of data. For applied research, there are typically two choices:
Locally-trained embeddings
- Collect a large corpus
- Estimate a word-embedding model
- Use the word-embeddings
- Advantages: Can capture “local” meanings of words which may differ from more general use
- Distadvantages: More computationally expensive and requires more coding decisions/effort
Pre-trained embeddings
- Download a pre-trained set of word-embeddings
- Use the word-embeddings
- Advantages: Usually high-quality embeddings trained on billions of texts
- Disadvantages: May miss local variation in word meaning
Rodriguez and Spirling, 2020 suggest that pre-trained embeddings normally perform equally well as locally-trained variants on most tasks.
Visualisation
It is common to see visualisations of word embeddings in lower dimensions
There are many approaches to dimensionality reduction
- Principal Component Analysis (PCA)
- t-distributed stochastic neighbour embeddings (t-SNE)
These visualisations often give helpful insights into the ways that language is used in the data that was used to train the models
Break
Using Word-Embeddings
Similarity
A key advantage of word embeddings: we can compute the similarity between words (or collections of words)
The similarity between two words can be calculated as the cosine of the angle between the embedding vectors:
\[cos(\theta) = \frac{\mathbf{w}_i \cdot \mathbf{w}_j}{\left|\left| \mathbf{w}_i \right|\right| \left|\left| \mathbf{w}_j \right|\right|}\]
- We can then sort the words in order of their similarity with the target word and report the “nearest neighbours”
Similarity Demonstration
taxes tax income paying taxation pay revenues fees excise costs
1.0000000 0.8410683 0.6800698 0.6292870 0.6281890 0.6165617 0.5964931 0.5957133 0.5911929 0.5867159 quantitative qualitative empirical measurement analysis methodology analytical analyses methodologies numerical
1.0000000 0.6452297 0.5144293 0.4902190 0.4807779 0.4792444 0.4602726 0.4536292 0.4420934 0.4269905 Analogies
One surprising feature of word-embeddings is that they can capture more nuanced features of language than simple similarity
Among the most widely discussed features of word embeddings is their ability to capture analogies via their geometry
Analogies are linguistic expressions which describe processes of transfering information from one subject (the analogue) to another (the target)
Example:
- Apple is to tree as grape is to ____
- King is to man as _____ is to woman
- Word embeddings have some ability to “solve” analogies of this form using vector addition and subtraction
\[\text{vector(king)} - \text{vector(man)} + \text{vector(woman)} \approx \text{vector(queen)}\]
Example process:
Compute vector \(\text{vector(king)} - \text{vector(man)} + \text{vector(woman)}\)
Calculate cosine similarity between new vector and all word vectors
Report most similar vectors (normally excluding those for king, man and woman)
Analogies Demonstration
# Extract vectors
king <- glove[which(rownames(glove) == "king"),]
man <- glove[which(rownames(glove) == "man"),]
woman <- glove[which(rownames(glove) == "woman"),]
# Generate analogy vector
target <- king - man + woman
# Calculate cosine similarity with all other vectors
target_sim <- sim2(glove,
matrix(target, nrow = 1))
# Print output
sort(target_sim[,1], decreasing = T)[1:10] king queen monarch throne princess mother daughter kingdom prince elizabeth
0.8065858 0.6896163 0.5575491 0.5565375 0.5518684 0.5142154 0.5133157 0.5025345 0.5017740 0.4908031 # Extract vectors
paris <- glove[which(rownames(glove) == "paris"),]
france <- glove[which(rownames(glove) == "france"),]
germany <- glove[which(rownames(glove) == "germany"),]
# Generate analogy vector
target <- paris - france + germany
# Calculate cosine similarity with all other vectors
target_sim <- sim2(glove,
matrix(target, nrow = 1))
# Print output
sort(target_sim[,1], decreasing = T)[1:10] berlin frankfurt germany munich cologne bonn vienna hamburg leipzig german
0.8082348 0.7182159 0.6976348 0.6616810 0.6388244 0.6297188 0.6096600 0.6015804 0.5951980 0.5929443 # Extract vectors
taller <- glove[which(rownames(glove) == "taller"),]
tall <- glove[which(rownames(glove) == "tall"),]
thin <- glove[which(rownames(glove) == "thin"),]
# Generate analogy vector
target <- taller - tall + thin
# Calculate cosine similarity with all other vectors
target_sim <- sim2(glove,
matrix(target, nrow = 1))
# Print output
sort(target_sim[,1], decreasing = T)[1:10] thinner thin thicker taller slimmer narrower noticeably softer slightly weaker
0.6823011 0.6795648 0.6409053 0.4766686 0.4659788 0.4645112 0.4491847 0.4438605 0.4299319 0.4269721 Implication: Word-embedding vectors encode certain linguistic regularities that relate to the relation between different words.
Caveats:
Only works with reasonably common words
Only works for certain relations, but not others
Understanding analogy is an open area for research
Dictionary Expansion
One helpful application of the word-similarity properties we have just discussed is that we can use them to automatically build more complete dictionaries
Process
Start with a small “seed” dictionary
Calculate the average embedding of the words in the dictionary
Calculate the cosine similarity between the dictionary embedding and all other words
Report the most similar words and use them to extend the original dictionary
This approach can enable us to find words associated with our concept of interest but which may not occur to the research a priori
Dictionary Expansion Application
# Define seed dictionary
seed_dictionary <- c("hate", "dislike", "despise")
# Extract seed words
hate_words <- glove[which(rownames(glove) %in% seed_dictionary),]
# Calculate mean embedding
hate_words_vec <- colMeans(hate_words)
# Calculate cosine similarity with all other vectors
target_sim <- sim2(glove,
matrix(hate_words_vec, nrow = 1))
# Print output
names(sort(target_sim[,1], decreasing = T))[1:40] [1] "despise" "dislike" "hate" "loathe" "hatred" "hated" "detest" "hates" "disdain" "distrust" "adore" "resent" "disliked" "distaste" "hating" "dislikes" "admire" "loathing" "feelings" "bigotry" "antipathy"
[22] "affection" "envy" "animosity" "intolerance" "equate" "abhor" "despises" "hostility" "despised" "profess" "admiration" "criticize" "liking" "fear" "perceive" "disrespect" "mistrust" "disliking" "hateful" Applications
Application 1
Are female politicians less aggressive than male politicians? (Hargrave and Blumenau, 2022)
In lecture 2 we investigated the claim that male and female politicians have distinct styles. Previously, we applied an existing sentiment dictionary to a corpus of parliamentary texts. Today, we will supplement this approach by using word-embeddings to automatically expand the set of words we use to score speeches.
Aggressive Word Dictionary
[1] "irritated" "stupid" "stubborn" "accusation" "acuse" "accusations" "accusing" "anger" "angered" "annoyance" "annoyed" "attack" "insult" "insulting"
[15] "insulted" "betray" "betrayed" "blame" "blamed" "blaming" "bitter" "bitterly" "bitterness" "complain" "complaining" "confront" "confrontation" "fibber"
[29] "fabricator" "phony" "fibber" "sham" "deceived" "deceive" "disgrace" "villain" "good-for-nothing" "hypocrite" "deception" "steal" "needlessly" "needless"
[43] "criticise" "criticised" "criticising" "blackened" "fiddled" "fiddle" "problematic" "lawbreakers" "offenders" "offend" "unacceptbale" "leech" "phoney" "appalling"
[57] "incapable" "farcical" "absurd" "ludicrous" "nonsense" "laughable" "nonsensical" "ridiculous" "outraged" "hysterial" "adversarial" "aggressive" "shady" "stereotyping"
[71] "unhelpful" "unnatural" "assaulted" "assault" "assaulting" "half-truths" "petty" "humiliate" "humiliating" "confrontational" "hate" "hatred" "furious" "hostile"
[85] "hostility" "nasty" "obnoxious" "sleeze" "sleezy" "inadequacy" "faithless" "neglectful" "neglect" "neglected" "wrong" "failure" "failures" "failed"
[99] "fail" "scapegoat" "cruel" "cruelty" "demonise" "demonised" "tactic" "trick" "trickery" "deceit" "dishonest" "deception" "devious" "deviouness"
[113] "shenanigans" "fraudulence" "fraudulent" "fraud" "swindling" "archaic" "sly" "slyness" "silly" "silliness" "scandal" "scandalous" "slander" "slanderous"
[127] "libellous" "disreputable" "dishonourable" "shameful" "atrocious" "gimmick" "immoral" "ridicule" "antagonistic" "antagonise" "ill-mannered" "spiteful" "spite" "vindictive"
[141] "prejudice" "prejudices" "disregard" "arrogant" "arrogance" "embarrasment" "embarrass" "embarrasing" "distasteful" "provoke" "provoked" "petulant" "ignorance" "stupidity"
[155] "idiot" "idiotic" "annoying" "dodgy" "untrue" "penny-pinching" "attacking" "ironic" "irony" "outrageous" "hackery" "crass" "backchat" "rude"
[169] "ill-judged" "ragbag" "mess" "hash" "fiasco" "shambles" "shambolic" "farce" "botch" "botched" "blunder" "mischievous" "mischief" "undermine"
[183] "straightjacket" "groan" "abuse" "chaos" "chaotic" "dull" "predictable" "negligent" "grotesque" "scapegoats" "hypocrisy" "bogus" "counterproductive" "betrayal"
[197] "patronise" "patronising" "reprehensible" "fool" "foolish" "abysmal" "disgraceful" "woeful" "inferior" "sneaky" "scaremongering" "scaremonger" "coward" "cowardly"
[211] "ignorant" "intolerant" "unacceptbale" "condemn" "short-sighted" "ashamed" "falsehood" "blackmail" "clownery" "debased" "debase" "hypocracy" "mislead" "misleading"
[225] "smokescreen" "subterfuge" "horrendous" "despicable" "deplorable" Although this is a reasonable-looking list of aggressive words, are there other words that MPs might use to criticise each other in parliamentary debate?
Estimating Word Embeddings
In this instance, we will use the Glove model to estimate a local set of word-embeddings
The hope is that this will allow us to pick up on the ways in which aggressive words are used in the specific context of parliamentary debate
library(text2vec)
# Load data
load("debates_fcm.Rdata")
## Fit GLOVE model
glove = GlobalVectors$new(rank = 150, x_max = 2500L)
debate_main = glove$fit_transform(debates_fcm, n_iter = 500, convergence_tol = 0.005, n_threads = 3, learning_rate = 0.14)
## Extract word embeddings
debate_context = glove$components
word_vectors = debate_main + t(debate_context)
save(word_vectors, file = "word_vectors_150.Rdata")Incorporating Word Embeddings
With our word-embeddings in hand, we can then use them to create a dictionary embedding by averaging over the embeddings for each word:
## Extract word embeddings of words in dictionary
target_words <- word_vectors[aggression_words,]
## Calculate mean embedding for this dictionary
target_vector <- colMeans(target_words)
## Distance between each word in the vocabulary and the mean embedding
cos_sim <- sim2(word_vectors,
matrix(target_vector, nrow = 1))
## Store results
word_scores <- data.frame(score = cos_sim[,1],
in_original_dictionary = dimnames(cos_sim)[[1]]%in%aggression_words)Incorporating Word Embeddings
score in_original_dictionary
disgraceful 0.6828765 TRUE
shameful 0.6611104 TRUE
outrageous 0.6553395 TRUE
scaremongering 0.6348777 TRUE
utterly 0.6147277 FALSE
cynical 0.6142637 FALSE
frankly 0.6091110 FALSE
scandalous 0.6077674 TRUE
dishonest 0.6039909 TRUE
embarrassing 0.5921001 FALSE
absurd 0.5897929 TRUE
ridiculous 0.5887514 TRUE
ludicrous 0.5873914 TRUE
deplorable 0.5846311 TRUE
incompetence 0.5773249 FALSE
misguided 0.5683095 FALSE
irresponsible 0.5675159 FALSE
pathetic 0.5667197 FALSE
appalling 0.5536031 TRUE
dreadful 0.5514060 FALSE
nonsense 0.5435853 TRUE
bizarre 0.5403646 FALSE
complacency 0.5319133 FALSE
ashamed 0.5275484 TRUE
illogical 0.5250856 FALSE
arrogant 0.5205944 TRUE
incompetent 0.5176657 FALSE
shocking 0.5158744 FALSE
accusation 0.5158350 TRUE
arrogance 0.5152852 TRUEScoring Speeches
- In addition to using this approach to finding words we might have missed, we now have scores associated with each word that indicate the relevance of the word to the concept of interest
- We can use these word-weights to score individual speeches
\[Score_i = \frac{\sum_w^W Sim_w N_{w,i}}{\sum_w^W N_{w,i}}\] - \(Sim_w\) is the similarity score for each word embedding and the dictionary embedding
\(N_{w,i}\) is the tf-idf count of the word in the speech
\(Score_i\) therefore represents the fraction of words in sentence \(i\) that are relevant to the concept contained in the seed dictionary
- Key advantage: speech scores reflect the ways that aggressive words are used in the context of parliamentary debate
Comparison with Traditional Dictionaries
Application 2
How do words change in meaning over time? (Hamilton et. al., 2018)
Understanding how words change their meanings over time is key to models of language and cultural evolution, but historical data on meaning is scarce, making theories hard to develop and test. Hamilton et. al. estimate (co-occurence vectors and Word2Vec) word-embeddings on the Google Books corpus to evaluate changes in word-meaning over 2 centuries.
- Frequent words change meaning at a slow rate, rare words change meaning faster
- Words with multiple meanings change at a fast rate, words with single meanings change slower
Extensions
Bias in Word-Embeddings
- An important substantive finding about word-embedding methods is that they can learn human biases in the semantic relationsips they encode into the vector space
- This occurs because they are trained on human-generated data: if biased relations between words occur frequently in natural language texts, the word-embeddings learn those biases
“There is nothing about doing data analysis that is neutral. What and how data is collected, how the data is cleaned and stored, what models are constructed, and what questions are asked – all of this is political.” Danah Boyd, NYU
- Another important theme of current work is in “de-biasing” word-embedding methods
Bias in Word-Embeddings, Example
# Extract vectors
doctor <- glove[which(rownames(glove) == "doctor"),]
father <- glove[which(rownames(glove) == "father"),]
mother <- glove[which(rownames(glove) == "mother"),]
# Generate analogy vector
target <- (doctor - father) + mother
# Calculate cosine similarity with all other vectors
target_sim <- sim2(glove,
matrix(target, nrow = 1))
# Print output
sort(target_sim[,1], decreasing = T)[1:10] doctor nurse doctors woman patient mother physician pregnant hospital medical
0.8397708 0.6648028 0.6255664 0.5923487 0.5839312 0.5719679 0.5527085 0.5417390 0.5404372 0.5336439 Racial Bias in Word-Embeddings (Garg et al., 2018, PNAS)
Polysemes
- A polyseme is a word or phrase that has multiple meanings
For example:
Pike
- A sharp point or staff
- A type of elongated fish
- A railroad line
- The future (coming down the pike)
- A type of body position (as in diving)
- Etc
- We typically have a single word-embedding vector for each word, which will be a (weighted) average of the contexts in which these different usages occur
- Contextual Word Embeddings (BERT; ELMo) are one potential solution: estimate a separate vector for every token, not just every type
Contextual Word Embeddings
Rodriguez et. al., 2023 provide a demonstration of combining regression methods with contextual word-embeddings to answer substantive social science research questions.
| Equality: 1885 | Equality: 2005 |
|---|---|
| enactment | gender |
| abolition | gays |
| slavery | lesbians |
| amendment | transgender |
| abrogation | lgbt |
| Trump | trump |
|---|---|
| president | declarer |
| impeaching | trumps |
| assailing | colloquies |
| president-elect | four-point |
| impeach | upend |
Contextual Embeddings and Modern NLP Methods
Modern NLP models (which we will continue to study over the next three weeks) are all built upon the idea of contextual embeddings which learn word meanings dynamically based on context.
| Traditional Embeddings | Contextual Embeddings |
|---|---|
| One vector per word | Different vectors per word use |
| Meaning is fixed | Meaning depends on sentence |
| Word similarity is static | Word relationships change dynamically |
Conclusion
Summing Up
Word embedding methods provide a representation of word “meaning” by encoding information about the contexts in which words occur
These vector result in a rich representation that allow us to measure the similarity between different words
There are several modelling decisions to make when estimating word embeddings, including modelling approach, context-window size, and embedding length
Embeddings improve many NLP tasks – classification, topic modelling, etc – by allowing generalisation and semantic reasoning
Over the next three weeks, embeddings will be a key building block of more advanced models
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