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Project 1 Unigram Model

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Machine Learning (B555)
Programming Project 1

Programming language and libraries: You can write your code in any programming language
so long as we are able to test it on SICE servers (python should be ok; please ask about other
options). We plan to run some or all or submitted code for further testing and validation.
You may use standard I/O, math, and plotting libraries. However, other than these, please write
all the code yourself without referring to special libraries or modules, i.e., no scikit, no pandas, no
other data processing libraries etc.
Data
Data for this assignment is provided in a zip file pp1data.zip on Canvas.
Overview: Unigram Model
Recall the unigram model from question 2 of written assignment 1. A unigram model over a
vocabulary of K words is specified by a discrete distribution with parameter µ. Under this model,
the probability of the k-th word of the vocabulary appearing is given by µk. In this experiment we
will use the unigram model to evaluate different learning methods, and to perform model selection.
Task 1: Model Training, Prediction, & Evaluation
In class we developed three methods for prediction: (1) using the maximum likelihood estimate,
(2) using the MAP estimate, and (3) using the predictive distribution. In this part, we evaluate the
effectiveness of these methods. More specifically, we will use text training data to perform unigram
model learning according to each method and then calculate the perplexity of the learned models
on a held-out test data set. Perplexity is a standard effectiveness metric in probabilistic language
modeling for scoring how well a model predicts a given collection of words (low perplexity values
imply good performance). Perplexity is defined by
P P = p(w1, w2, · · · , wN |model)− 1
N
unigram
= exp

1
N
X
N
i=1
ln p(wi)
!
1
In this part we use the data files training data.txt and test data.txt each of size N = 640, 000
words. We have pre-processed and “cleaned” the text. While there are many obvious further
potential improvements, for uniformity and ease of grading, please do not perform any further
manipulation of the data – for this assignment you only have to read the space-separated strings
from the corresponding ASCII text files.
For each size of training set in  N
128 ,
N
64 ,
N
16 ,
N
4
, N
, you should train a unigram model according
to the three different methods (use the initial segment of the full training data). Use a Dirichlet
distribution with parameter α = α
01 as a prior (this is a scalar α
0 multiplied by a vector of ones)
and set α
0 = 2 for this part. Prediction equations for these models are given below.
To avoid having words in the test set that are not in your vocabulary start by building a dictionary
from the entire train and test sets and use this vocabulary in the experiments. You should find
K=10000 distinct words.
When run, your code should report the perplexities on the train set (i.e., the current portion being
trained on) and test set under the three trained models for each train set size. Plot the results as
a function of train set size (it is insightful to put all three methods together in the same plot) and
provide some observations. In your discussion of the results, please address the following:
• What happens to the test set perplexities of the different methods with respect to each other
as the training set size increases? Please explain why this occurs.
• What is the obvious shortcoming of the maximum likelihood estimate for a unigram model?
How do the other two approaches address this issue?
• For the full training set, how sensitive do you think the test set perplexity will be to small
changes in α
0
? why?
Task 2: Model Selection
Here we use the same data and dictionary K=10000 as in task 1. In the previous part we set the
value of the hyperparameter, α
0
, manually. In this part, you will use the evidence function (from
written assignment 1) and training data to select a value of α
0
. In general, direct maximization
of the evidence function to determine the hyperparameter can be difficult. Here we only have one
hyperparameter α
0 and can therefore use a “brute-force” grid search to select its value.
In particular, compute the log evidence at α
0 = 1.0, 2.0, . . . , 10.0 for a training set of size N
128 . Also,
compute the perplexities on the test set (use the predictive distribution) at these same values.
When run your code should output the list of log-evidence and perplexity values for each α.
Plot the log evidence and test set perplexity as a function of α
0 and discuss what you see. In your
discussion of the results, please address the following:
• Is maximizing the evidence function a good method for model selection on this dataset?
2
Task 3: Author Identification
Can the unigram model identify authors? In this part, we apply the model to this problem. On
the course web page you will find 3 additional files for this assignment. Each of them is a cleaned
text version of a classic novel (thanks to the Gutenberg project).
To avoid having words in the test set that are not in your vocabulary start by building a dictionary
from all 3 files and use this vocabulary in the experiments. You should find K=18,251 distinct
words.
Train the model on pg121.txt.clean (use the predictive distribution with α
0 = 2) and evaluate
the perplexity on each of the other two texts. When run your code should output the perplexities
for the two other texts. One of the test files is by the same author as the training file but the other
is not (121 and 141 are by J. Austen and 1400 is by C. Dickens). Was the model successful in this
classification task? Please report and discuss these results.
Additional Notes
• In Task 1, you will need to handle ln(0) , −∞ as a special case in your code. Please do
not smooth these values as that will distort the results. When plotting you can drop infinity
values or cap them with an appropriate large number that preserves visibility.
• Useful Equations:
– Prediction using ML estimate: p(next word = k-th word of vocabulary) = mk
N
– Prediction using MAP estimate: p(next word = k-th word of vocabulary) = mk+αk−1
N+α0−K
– Prediction using predictive distribution: p(next word = k-th word of vocabulary) =
mk+αk
N+α0
– Evidence: Pr(Data|α) = Γ(α0)
QK
k=1 Γ(αk+mk)
Γ(α0+N)
QK
k=1 Γ(αk)
– In the above, mk = Number of times the k-th word of the vocabulary appears in the training documentN = Total number of words in the training document, and α0 =
PK
k=1 αk.
• Recall that Γ(x) = (x−1)!. Since our x values are integers you can calculate Γ() values using
factorial. To avoid numerical overflow, instead of calculating the factorial and then taking
log, calculate the log directly.
• For your reference and help in debugging, the values of test set perplexity to expect in Task
1 with sufficiently large dataset are in the range 9,000± 2000.
Submission
Please submit two separate items via Canvas:
(1) A zip file pp1.zip with all your work and the report. The zip file should include: (1a) Please
write a report on the experiments, include all plots and results, and your conclusions as requested
above. Prepare a PDF file with this report. (1b) Your code for the assignment, including a
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README file that explains how to run it. When run your code should produce all the results and
plots as requested above. Your code should assume that the data files will have names as specified
above and will reside in sub-directory pp1data/ of the directory where the code is executed. We
will read your code as part of the grading – please make sure the code is well structured and easy
to follow (i.e., document it as needed). This portion can be a single file or multiple files.
(2) One PDF “printout” of all contents in 1a,1b: call this YourName-pp1-everything.pdf. One
PDF file which includes the report, a printout of the code and the README file. We will use
this file as a primary point for reading your submission and providing feedback so please include
anything pertinent here.
Grading
Your assignment will be graded based on (1) the clarity of the code, (2) its correctness, (3) the
presentation and discussion of the results, (4) The README file and our ability to follow the
instructions and test the code.
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