Description
Introduction
This assignment will give you experience with a social media corpus (i.e., a collection of posts from Reddit),
Python programming, part-of-speech (PoS) tags, sentiment analysis, and machine learning with scikit-learn.
Your task is to split posts into sentences, tag them with a PoS tagger that we will provide, gather some
feature information from each post, learn models, and use these to classify political persuasion. Sentiment
analysis is an important topic in computational linguistics in which we quantify subjective aspects of
language. These aspects can range from biases in social media for marketing, to a spectrum of cognitive
behaviours for disease diagnosis.
Please check the course bulletin board for announcements and discussion pertaining to this assignment.
Reddit Corpus
We have curated data from Reddit by scraping subreddits, using Pushshift, by perceived political affiliation.
Table 1 shows the subreddits assigned to each of four categories: left-leaning, right-leaning, center/neutral,
and ‘alternative facts’. Although the first three (at least) are often viewed as ordinal segments on a
unidimensional spectrum, here we treat these categories as nominal classes. Here, we use the terms ‘post’
and ‘comment’ interchangeably to mean ‘datum’, i.e., a short segment of user-produced text.
Left (598, 944) Center (599, 872) Right (600, 002) Alt (200, 272)
twoXChromosomes (7, 720, 661) news (2, 782, 9911) theNewRight (19, 466) conspiracy (6, 767, 099)
occupyWallStreet (397, 538) politics (60, 354, 767) whiteRights (118, 008) 911truth (79, 868)
lateStageCapitalism (634, 962) energy (416, 926) Libertarian (3, 886, 156)
progressive (246, 435) canada (7, 225, 005) AskTrumpSupporters (1, 007, 590)
socialism (1, 082, 305) worldnews (38, 851, 904) The Donald (21, 792, 999)
demsocialist (5269) law (464, 236) new right (25, 166)
Liberal (151, 350) Conservative (1, 929, 977)
tea party (1976)
Table 1: Subreddits assigned to each category, with the total posts in each. Since there are over 181M
posts, we sample randomly within each category – the resulting number of available posts for each category
in this assignment is shown on the top row.
These data are stored on the teach.cs servers under /u/cs401/A1/data/. To save space, these files
should only be accessed from that directory (and not copied). All data are in the JSON format.
Copyright
c 2019, Frank Rudzicz, Chlo´e Pou-Prom. All rights reserved.
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Each datum has several fields of interest, including:
ups: the integer number of upvotes.
downs: the integer number of downvotes.
score: [ups − downs]
controversiality: a combination of the popularity of a post and the ratio between ups and downs.
subreddit: the subreddit from which the post was sampled.
author: the author ID.
body: the main textual message of the post, and our primary interest.
id: the unique identifier of the comment.
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Your tasks
1 Pre-processing, tokenizing, and tagging [25 marks]
The comments, as given, are not in a form amenable to feature extraction for classification – there is
too much ‘noise’. Therefore, the first step is to complete a Python program named a1 preproc.py, in
accordance with Section 5, that will read subsets of JSON files, and for each comment perform the following
steps, in order, on the ‘body’ field of each selected comment:
1. Remove all newline characters.
2. Replace HTML character codes (i.e., &…;) with their ASCII equivalent (see http://www.asciitable.com).
3. Remove all URLs (i.e., tokens beginning with http or www).
4. Split each punctuation (see string.punctuation) into its own token using whitespace except:
• Apostrophes.
• Periods in abbreviations (e.g., e.g.) are not split from their tokens. E.g., e.g. stays e.g.
• Multiple punctuation (e.g., !?!, …) are not split internally. E.g., Hi!!! becomes Hi !!!
• You can handle single hyphens (-) between words as you please. E.g., you can split non-committal
into three tokens or leave it as one.
5. Split clitics using whitespace.
• Clitics are contracted forms of words, such as n’t, that are concatenated with the previous word.
• Note: the possessive ’s has its own tag and is distinct from the clitic ’s, but nonetheless must
be separated by a space; likewise, the possessive on plurals must be separated (e.g., dogs ’).
6. Each token is tagged with its part-of-speech using spaCy (see below).
• A tagged token consists of a word, the ‘/’ symbol, and the tag (e.g., dog/NN). See below for
information on how to use the tagging module. The tagger can make mistakes.
7. Remove stopwords. See /u/cs401/Wordlists/StopWords.
8. Apply lemmatization using spaCy (see below).
9. Add a newline between each sentence.
• This will require detecting end-of-sentence punctuation. Some punctuation does not end a
sentence; see standard abbreviations here: /u/cs401/Wordlists/abbrev.english. It can
be difficult to detect when an abbreviation ends a sentence; e.g., in Go to St. John’s St. John
is there., the first period is used in an abbreviation, the last period ends a sentence, and the
second period is used both in an abbreviation and an end-of-sentence. You are not expected to
write a ‘perfect’ pre-processor (none exists!), but merely to use your best judgment in writing
heuristics; see section 4.2.4 of the Manning and Sch¨utze text for ideas.
10. Convert text to lowercase.
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Functionality: The a1 preproc.py program reads a subset of the (static) input JSON files, retains
the fields you care about, including ‘id’, which you’ll soon use as a key to obtain pre-computed features,
and ‘body’, which is text that you preprocess and replace before saving the result to an output file. To
each comment, also add a cat field, with the name of the file from which the comment was retrieved (e.g.,
‘Left’, ‘Alt’,…).
The program takes three arguments: your student ID (mandatory), the output file (mandatory), and
the maximum number of lines to sample from each category file (optional; default=10,000). For example,
if you are student 999123456 and want to create preproc.json, you’d run:
python a1 preproc.py 999123456 -o preproc.json
The output of a1 preproc.py will be used in Task 2.
Your task: Copy the template from /u/cs401/A1/code/a1 preproc.py. There are two functions you
need to modify:
1. In preproc1, fill out each if statement with the associated preprocessing step above.
2. In main, replace the lines marked with TODO with the code they describe.
For this section, you may only use standard Python libraries, except for tagging (step 6) and lemmatization (step 8), as described below. For debugging, you are advised to either use a different input folder
with your own JSON data, or pass strings directly to preproc1.
spaCy: spaCy is a Python library for natural language processing tasks, especially in information
extraction. Here, we only use its ability to obtain part-of-speech tags and lemma. For example:
import spacy
nlp = spacy.load(’en’, disable=[’parser’, ’ner’])
utt = nlp(u”I know the best words”)
for token in utt:
… print(token.text, token.lemma_, token.pos_, token.tag_, token.dep_,
… token.shape_, token.is_alpha, token.is_stop)
When performing step (6) above, simply append ‘/’ and the appropriate token.tag to the end of each
tag, as per the earlier example.
When performing step (8) above, simply replace the token itself with the token.lemma . E.g., words/NNS
becomes word/NNS. If the lemma begins with a dash (‘-’) when the token doesn’t (e.g., -PRON- for I, just
keep the token.
Subsampling: By default, you should only sample 10,000 lines from each of the Left, Center, Right,
and Alt files, for a total of 40,000 lines. From each file, start sampling lines at index [ID % len(X)], where
ID is your student ID, % is the modulo arithmetic operator, and len(X) is the number of comments in
the given input file (i.e., len(data), once the JSON parse is done). Use circular list indexing if your start
index is too close to the ‘end’.
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2 Feature extraction [20 marks]
The second step is to complete a Python program named a1 extractFeatures.py, in accordance with
Section 5, that takes the preprocessed comments from Task 1, extracts features that are relevant to bias
detection, and builds an npz datafile that will be used to train models and classify comments in Task 3.
For each comment, you need to extract 173 features and write these, along with the category, to a single
NumPy array. These features are listed below. Several of these features involve counting tokens based on
their tags. For example, counting the number of adverbs in a comment involves counting the number of
tokens that have been tagged as RB, RBR, or RBS. Table 4 explicitly defines some of these features; other
definitions are available on CDF in /u/cs401/Wordlists/. You may copy and modify these files, but do
not change their filenames.
1. Number of first-person pronouns
2. Number of second-person pronouns
3. Number of third-person pronouns
4. Number of coordinating conjunctions
5. Number of past-tense verbs
6. Number of future-tense verbs
7. Number of commas
8. Number of multi-character punctuation tokens
9. Number of common nouns
10. Number of proper nouns
11. Number of adverbs
12. Number of wh- words
13. Number of slang acronyms
14. Number of words in uppercase (≥ 3 letters long)
15. Average length of sentences, in tokens
16. Average length of tokens, excluding punctuation-only tokens, in characters
17. Number of sentences.
18. Average of AoA (100-700) from Bristol, Gilhooly, and Logie norms
19. Average of IMG from Bristol, Gilhooly, and Logie norms
20. Average of FAM from Bristol, Gilhooly, and Logie norms
21. Standard deviation of AoA (100-700) from Bristol, Gilhooly, and Logie norms
22. Standard deviation of IMG from Bristol, Gilhooly, and Logie norms
23. Standard deviation of FAM from Bristol, Gilhooly, and Logie norms
24. Average of V.Mean.Sum from Warringer norms
25. Average of A.Mean.Sum from Warringer norms
26. Average of D.Mean.Sum from Warringer norms
27. Standard deviation of V.Mean.Sum from Warringer norms
28. Standard deviation of A.Mean.Sum from Warringer norms
29. Standard deviation of D.Mean.Sum from Warringer norms
30-173. LIWC/Receptiviti features
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Functionality: The a1 extractFeatures.py program reads a preprocessed JSON file and extracts
features for each comment therein, producing and saving a D × 174 NumPy array, where the i
th row is the
features for the i
th comment, followed by an integer for the class (0: Left, 1: Center, 2: Right, 3: Alt), as
per the cat JSON field.
The program takes two arguments: the input filename (i.e., the output of a1 preproc), and the output
filename. For example, given input preproc.json and the desired output feats.npz, you’d run:
python a1 extractFeatures.py -i preproc.json -o out.json
The output of a1 extractFeatures.py will be used in Task 3.
Your task: Copy the template from /u/cs401/A1/code/a1 extractFeatures.py. There are two
functions you need to modify:
1. In extract1, extract each of the 173 aforementioned features from the input string.
2. In main, call extract1 on each datum, and add the results (+ the class) to the feats array.
When your feature extractor works to your satisfaction, build feats.npz, from all input data.
Norms: Lexical norms are aggregate subjective scores given to words by a large group of individuals.
Each type of norm assigns a numerical value to each word. Here, we use two sets of norms:
Bristol+GilhoolyLogie: These are found in /u/cs401/Wordlists/BristolNorms+GilhoolyLogie.csv,
specifically the fourth, fifth, and sixth columns. These measure the Age-of-acquisition (AoA), imageability (IMG), and familiarity (FAM) of each word, which we can use to measure lexical complexity.
More information can be found, for example, here.
Warringer: These are found in /u/cs401/Wordlists/Ratings Warriner et al.csv, specifically the
third, sixth, and ninth columns. These norms measure the valence (V), arousal (A), and dominance
(D) of each word, according to the VAD model of human affect and emotion. More information on
this particular data set can be found here.
When you compute features 18-29, only consider those words that exist in the respective norms file.
LIWC/Receptiviti: The Linguistic Inquiry & Word Count (LIWC) tool has been a standard in
a variety of NLP research, especially around authorship and sentiment analysis. This tool provides 85
measures mostly related to word choice; more information can be found here. The company Receptiviti
provides a superset of these features, which also includes 59 measures of personality derived from text.
The company has graciously donated access to its API for the purposes of this course.
To simplify things, we have already extracted these 144 features for you. Simply copy the pre-computed
features from the appropriate uncompressed npy files stored in /u/cs401/A1/feats/. Specifically:
1. Comment IDs are stored in IDs.txt files (e.g., Alt IDs.txt). When processing a comment, find the
index (row) i of the ID in the appropriate ID text file, for the category, and copy the 144 elements,
starting at element i · 144, from the associated feats.dat.npy file.
2. The file feats.txt provides the names of these features, in the order provided. For this assignment,
these names will suffice as to their meaning, but you are welcome to obtain your own API license
from Receptiviti in order to get access to their documentation.
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3 Experiments and classification [30 marks]
The third step is to use the features extracted in Task 2 to classify comments using the scikit-learn machine
learning package. Here, you will modify various hyper-parameters and interpret the results analytically. As
everyone has different slices of the data, there are no expectations on overall accuracy, but you are expected
to discuss your findings with scientific rigour. Copy the template from /u/cs401/A1/code/a1 classify.py
and complete the main body and the functions for the following experiments according to the specifications
therein:
3.1 Classifiers
Use the train test split method to split the data into a random 80% for training and 20% for testing.
Train the following 5 classifiers (see hyperlinks for API) with fit(X train, y train):
1. SVC: support vector machine with a linear kernel.
2. SVC: support vector machine with a radial basis function (γ = 2) kernel.
3. RandomForestClassifier: with a maximum depth of 5, and 10 estimators.
4. MLPClassifier: A feed-forward neural network, with α = 0.05.
5. AdaBoostClassifier: with the default hyper-parameters.
Here, X train is the first 173 columns of your training data, and y train is the last column. Obtain
predicted labels with these classifiers using predict(X test), where X test is the first 173 columns of
your testing data. Obtain the 4 × 4 confusion matrix C using confusion matrix. Given that the element
at row i, column j in C (i.e., ci,j ) is the number of instances belonging to class i that were classified as
class j, compute the following manually, using the associated function templates:
Accuracy : the total number of correctly classified instances over all classifications: A =
P
i P ci,i
i,j ci,j
.
Recall : for each class κ, the fraction of cases that are truly class κ that were classified as κ, R(κ) = Pcκ,κ
j
cκ,j
.
Precision : for each class κ, the fraction of cases classified as κ that truly are κ, P(κ) = Pcκ,κ
i
ci,κ
.
Write the results to the text file a1 3.1.csv, in a comma-separated value format. Each of the first
five lines has the following, in order:
1. the number of the classifier (i.e., 1-5)
2. the overall accuracy, recall for the 4 classes, and precision for the 4 classes
3. the confusion matrix, read row-by-row
That is, each of the first five lines should have 1 + (1 + 4 + 4) + 4 × 4 = 26 numbers separated by commas.
If so desired, add any analytical commentary to the sixth line of this file.
3.2 Amount of training data
Many researchers attribute the success of modern machine learning to the sheer volume of data that is
now available. Modify the amount of data that is used to train your preferred classifier from above in five
increments: 1K, 5K, 10K, 15K, and 20K. These can be sampled arbitrarily from the training set in Section
3.1. Using only the classification algorithm with the highest accuracy from Section 3.1, report the resulting
accuracies in a comma-separated form in the first line of a1 3.2.csv. On the second line of that file,
comment on the changes to accuracy as the number of training samples increases, including at least two
sentences on a possible explanation. Is there an expected trend? Do you see such a trend? Hypothesize as
to why or why not.
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3.3 Feature analysis
Certain features may be more or less useful for classification, and too many can lead to overfitting or other
problems. Here, you will select the best features for classification using SelectKBest according to the
f classif metric as in:
from sklearn.feature_selection import SelectKBest
from sklearn.feature_selection import f_classif
selector = SelectKBest(f_classif, you_figure_it_out)
X_new = selector.fit_transform(X_train, y_train)
pp = selector.pvalues_
In the example above, pp stores the p-value associated with doing a χ
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statistical test on each feature. A
smaller value means the associated feature better separates the classes. Do this:
1. For each of the 1K training set from Section 3.2, and the original 32K training set from Section
3.1, and for each number of features k = {5, 10, 20, 30, 40, 50}, find the best k features according to
this approach. On each of the first 6 lines of a1 3.3.csv, write the number of features, and the
associated p-values for the 32K training set case, separated by commas.
2. Train the best classifier from section 3.1 for each of the 1K training set and the 32K training set,
using only the best k = 5 features. On the 7th line of a1 3.3.csv, write the accuracy for the 1K
training case and the 32K training case, separated by a comma
3. On lines 8 to 10 of a1 3.3.csv, answer the following questions:
(a) What features, if any, are chosen at both the low and high(er) amounts of input data? Also
provide a possible explanation as to why this might be.
(b) Are p-values generally higher or lower given more or less data? Why or why not?
(c) Name the top 5 features chosen for the 32K training case. Hypothesize as to why those particular
features might differentiate the classes.
3.4 Cross-validation
Many papers in machine learning stick with a single subset of data for training and another for testing
(occasionally with a third for validation). This may not be the most honest approach. Is the best classifier
from Section 3.1 really the best? For each of the classifiers in Section 3.1, run 5-fold cross-validation given
all the initially available data. Specifically, use KFold. Set the shuffle argument to true.
For each fold, obtain accuracy on the test partition after training on the rest for each classifier, in the
original order, and report these on their own line (one per fold) in a1 3.4.csv, separated by commas.
Next, compare the accuracies of your best classifier, across the 5 folds, with each of the other 4 classifiers
to check if it is significantly better than any others. I.e., given vectors a and b, one for each classifier,
containing the accuracy values for each of the respective 5 folds, obtain the p-value from the output S,
below:
from scipy import stats
S = stats.ttest_rel(a, b)
You should have 4 p-values. Report each in a1 3.4.csv, on the sixth line, separated by commas, in the
order the classifiers appear in Section 3.1. On the seventh line, comment on any significance you observe,
or any lack thereof, and hypothesize as to why, in one to three sentences.
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4 Bonus [15 marks]
We will give up to 15 bonus marks for innovative work going substantially beyond the minimal requirements.
These marks can make up for marks lost in other sections of the assignment, but your overall mark for
this assignment cannot exceed 100%. The obtainable bonus marks will depend on the complexity of the
undertaking, and are at the discretion of the marker. Importantly, your bonus work should not affect our
ability to mark the main body of the assignment in any way.
You may decide to pursue any number of tasks of your own design related to this assignment, although
you should consult with the instructor or the TA before embarking on such exploration. Certainly, the
rest of the assignment takes higher priority. Some ideas:
• Identify words that the PoS tagger tags incorrectly and add code that fixes those mistakes.
Does this code introduce new errors elsewhere? E.g., if you always tag dog as a noun to correct a
mistake, you will encounter errors when dog should be a verb. How can you mitigate such errors?
• Explore alternative features to those extracted in Task 2. What other kinds of variables would
be useful in distinguishing affect? Consider, for example, the Stanford Deep Learning for Sentiment
Analysis. Test your features empirically as you did in Task 3 and discuss your findings.
• Explore alternative classification methods to those used in Task 3. Explore different hyperparameters. Which hyper-parameters give the best empirical performance, and why?
• Learn about topic modelling as in latent Dirichlet allocation. Are there topics that have an effect
on the accuracy of the system? E.g., is it easier to tell how someone feels about politicians or about
events? People or companies? As there may be class imbalances in the groups, how would you go
about evaluating this? Go about evaluating this.
5 General specifications
As part of grading your assignment, the grader may run your programs and/or arff files on test data and
configurations that you have not previously seen. This may be partially done automatically by scripts. It
is therefore important that each of your programs precisely meets all the specifications, including its name
and the names of the files and functions that it uses. A program that cannot be evaluated because
it varies from specifications will receive zero marks on the relevant sections.
If a program uses a file, such as the word lists, whose name is specified within the program, the file
must be read either from the directory in which the program is being executed, or from a subdirectory
of /u/cs401 whose path is completely specified in the program. Do not hardwire the absolute address of
your home directory within the program; the grader does not have access to this directory.
All your programs must contain adequate internal documentation to be clear to the graders.
We use Python version 3.6.
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6 Submission requirements
This assignment is submitted electronically. You should submit:
1. All your code for a1 preproc.py, a1 extractFeatures.py, and a1 classify.py (including helper
files, if any).
2. a1 3.1.csv: Report on classifiers.
3. a1 3.2.csv: Report on the amount of training data.
4. a1 3.3.csv: Report on feature analysis.
5. a1 3.4.csv: Report on 5-fold cross-validation.
6. Any lists of words that you modified from the original version.
In another file called ID (use the template on the course web page), provide the following information:
1. your first and last name.
2. your student number.
3. your CDF/teach.cs login id.
4. your preferred contact email address.
5. whether you are an undergraduate or graduate.
6. this statement: By submitting this file, I declare that my electronic submission is my own work, and
is in accordance with the University of Toronto Code of Behaviour on Academic Matters and the
Code of Student Conduct, as well as the collaboration policies of this course.
You do not need to hand in any files other than those specified above. Submit your assignment
on MarkUs. Do not tar or compress your files, and do not place your files in subdirectories. Do not
format your discussion as a PDF or Word document — use plain text only.
7 Working outside the lab
If you want to do some or all of this assignment on your laptop or home computer, for example, you will
have to do the extra work of downloading and installing the requisite software and data. If you take this
route, you take on all associated risks. You are strongly advised to upload regular backups of your work to
CDF/teach.cs, so that if your home machine fails or proves to be inadequate, you can immediately continue
working on the assignment at CDF/teach.cs. When you have completed the assignment, you should try
your programs out on CDF/teach.cs to make sure that they run correctly there. Any component that
does not work on CDF will get zero marks.
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Appendix: Tables
Tag Name Example
CC Coordinating conjunction and
CD Cardinal number three
DT Determiner the
EX Existential there there [is]
FW Foreign word d’oeuvre
IN Preposition or subordinating in, of, like
conjunction
JJ Adjective green, good
JJR Adjective, comparative greener, better
JJS Adjective, superlative greenest, best
LS List item marker (1)
MD Modal could, will
NN Noun, singular or mass table
NNS Noun, plural tables
NNP Proper noun, singular John
NNPS Proper noun, plural Vikings
PDT Predeterminer both [the boys]
POS Possessive ending ’s, ’
PRP Personal pronoun I, he, it
PRP$ Possessive pronoun my, his, its
RB Adverb however, usually, naturally, here, good
RBR Adverb, comparative better
RBS Adverb, superlative best
RP Particle [give] up
SYM Symbol (mathematical or scientific) +
TO to to [go] to [him]
UH Interjection uh-huh
VB Verb, base form take
VBD Verb, past tense took
VBG Verb, gerund or present participle taking
VBN Verb, past participle taken
VBP Verb, non-3rd-person singular present take
VBZ Verb, 3rd-person singular present takes
WDT wh-determiner which
WP wh-pronoun who, what
WP$ Possessive wh-pronoun whose
WRB wh-adverb where, when
Table 2: The Penn part-of-speech tagset—words
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Tag Name Example
# Pound sign £
$ Dollar sign $
. Sentence-final punctuation !, ?, .
, Comma
: Colon, semi-colon, ellipsis
( Left bracket character
) Right bracket character
” Straight double quote
‘ Left open single quote
“ Left open double quote
’ Right close single quote
” Right close double quote
Table 3: The Penn part-of-speech tagset—punctuation
First person:
I, me, my, mine, we, us, our, ours
Second person:
you, your, yours, u, ur, urs
Third person:
he, him, his, she, her, hers, it, its, they, them, their, theirs
Future Tense:
’ll, will, gonna, going+to+VB
Common Nouns:
NN, NNS
Proper Nouns:
NNP, NNPS
Adverbs:
RB, RBR, RBS
wh-words :
WDT, WP, WP$, WRB
Modern slang acronyms:
smh, fwb, lmfao, lmao, lms, tbh, rofl, wtf, bff, wyd, lylc, brb, atm, imao, sml, btw,
bw, imho, fyi, ppl, sob, ttyl, imo, ltr, thx, kk, omg, omfg, ttys, afn, bbs, cya, ez, f2f,
gtr, ic, jk, k, ly, ya, nm, np, plz, ru, so, tc, tmi, ym, ur, u, sol, fml Consider also
https://www.netlingo.com/acronyms.php, if you want, for no-bonus completion.
Table 4: Miscellaneous feature category specifications.
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