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Project 2  Na¨ıve Bayes Classifier

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COMP 472/6721 Project 2
Purpose In this project, you will implement a Na¨ıve Bayes Classifier to use as a spam filter.
The project must be programmed in Python, and can only use the following libraries: NumPy, math, re, sys
and Matplotlib.
1 Your Task
Download the email corpus available on Moodle. This dataset contains emails that are already classified into 2
classes: ham and spam. You can assume that the files can be processed using latin-1 encoding.
The project will be divided into 3 tasks:
Task 1: Building the model.
Task 2: Evaluating and analyzing the classifier.
Task 3: Experimenting with the classifier.
1.1 Task 1: Building the Model
Write a Python program to build a probabilistic model from the training set. Your code will parse the files in the
training set and build a vocabulary with all the words it contains. Then for each word, compute their frequencies
and probabilities for each class (class ham and class spam).
To process the texts, fold them to lowercase, then tokenize them using re.split(’\[\^a-zA-Z\]’,aString)
and use the set of resulting words as your vocabulary.
For each word wi
in the training set, save its frequency and its conditional probability for each class:
P(wi
|ham) and P(wi
|spam). These probabilities must be smoothed using the add δ with δ = 0.5.
Save your model in a text file called model.txt. The format of this file must be the following:
1. A line counter i, followed by 2 spaces.
2. The word wi
, followed by 2 spaces.
3. The frequency of wi
in the class ham, followed by 2 spaces.
4. The smoothed conditional probability of wi
in the class ham – P(wi
|ham), followed by 2 spaces.
5. The frequency of wi
in the class spam, followed by 2 spaces.
6. The smoothed conditional probability of wi
in spam – P(wi
|spam), followed by a carriage return.
Note that the file must be sorted alphabetically. For example your file model.txt could look like the following:
1 abc 3 0.003 40 0.4
2 airplane 3 0.003 40 0.4
3 password 40 0.4 50 0.03
4 zucchini 0.7 0.003 0 0.000001
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1.2 Task 2: Building and Evaluating the Classifier
Once you have built your model (task 1), use it to implement and test a Na¨ıve Bayes classifier to classify emails
into their most likely class: ham or spam. To avoid arithmetic underflow, work in log10 space.
Run your classifier on the test set given and create a single file called baseline-result.txt with your
classification results. For each test file, baseline-result.txt should contain:
1. a line counter, followed by 2 spaces
2. the name of the test file, followed by 2 spaces
3. the classification as given by your classifier (the label spam or ham), followed by 2 spaces
4. the score of the class ham as given by your classifier, followed by 2 spaces
5. the score of the class spam as given by your classifier, followed by 2 spaces
6. the correct classification of the file, followed by 2 spaces
7. the label right or wrong (depending on the case), followed by a carriage return.
For example your result file could look like the following :
1 test-ham-00001.txt ham 0.004 0.001 ham right
2 test-ham-00002.txt spam 0.002 0.03 ham wrong
1.3 Task 3: Experiment with your Classifier
Now the fun part! Tasks 1 and 2 above will constitute your experiment 1, or baseline experiment, and you will
perform variations over this baseline to see if they improve the performance of your classifier.
1.3.1 Experiment 2: Stop-word Filtering
Download the list of stop words available on Moodle. Use the baseline experiment and redo tasks 1 and 2 but
this time remove the stop words from your vocabulary. Generate the new model and result files that you will
call stopword-model.txt and stopword-result.txt.
1.3.2 Experiment 3: Word Length Filtering
Use the baseline experiment and redo tasks 1 and 2 but this time remove all words with length ≤ 2 and all
words with length ≥ 9. Generate the new model and result files that you will call wordlength-model.txt and
wordlength-result.txt.
1.3.3 Experiment 4: Infrequent Word Filtering
Only COMP 6721 students need to do experiment 4. Use the baseline experiment, and gradually remove from
the vocabulary words with frequency= 1, frequency ≤ 5, frequency ≤ 10, frequency ≤ 15 and frequency ≤ 20.
Then gradually remove the top 5% most frequent words, the 10% most frequent words, 15%, 20% and 25% most
frequent words. Plot the performance of the classifier against the number of words left in your vocabulary.
1.3.4 Experiment 5: Smoothing
Only COMP 6721 students need to do experiment 5. Use the baseline experiment, and change the smoothing
value gradually from δ = 0 to δ = 1 in steps of 0.1. Plot the performance of the classifier against the smoothing
value.
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1.4 Report
1.4.1 Report for COMP 472
In COMP 472, your report should be 3-5 pages (without references and appendix) and use the template provided
on Moodle. The report should contain at least the following:

1
2
page: Show and analyze the results of the baseline experiment (exp. #1). Give a table of results showing
the accuracy, precision, recall and F1-measure for each class, as well as a confusion matrix. Analyze and
discuss these results.

1
2
page: Do the same with the results of the stop-word filtering experiment (exp. #2).

1
2
page: Do the same with the results of the word-length filtering experiment (exp. #3).
• 1 page: Compare & discuss the results of the 3 experiments.

1
2
page: Describe any difficulties that you have encountered and how you addressed them.

1
2
page: If you were to continue working on this project, what do you feel would be interesting to investigate?
Are there questions that you would like to investigate more, if you had the time and the energy?
• Your report should have a reference section (not included in the page count) that properly cites all relevant
resources that you have consulted (books, Web sites . . . ), even if it was just to inspire you. Failure to
properly cite your references constitutes plagiarism and will be reported.
Your report should:
follow the Word or LATEX template provided on Moodle or an equivalent format
be submitted in PDF format
be called 472 P2 Report StudentID1 StudentID2 …pdf
1.4.2 Report for COMP 6721
In COMP 6721, your report should be 4-6 pages. In addition to the content of the COMP 472 report (see
Section 1.4.1), your report should include:

1
2
page: Results and analysis of the infrequent word filtering experiment (exp. #4).

1
2
page: Results and analysis of the smoothing experiment (exp. #5)
• 1 page: Compare & discuss the results of the 5 experiments
Your report should:
follow the Word or LATEX template provided on Moodle or an equivalent format
be submitted in PDF format
be called 6721 P2 Report StudentID1 StudentID2 …pdf
1.5 Demos
The project will be demonstrated to the TAs in the lab. Regardless of the demo time, you will demo the program
that was uploaded as the official submission on or before the due date. The schedule of the demos will be posted
on Moodle. No special preparation is necessary for the demo (no slides or prepared speech). Your TA will ask
you questions on your code, and you will have to answer him/her. Part of your marks will be based on your
demos.
At the demo, the TA will give you a new dataset and you will need to generate new output files called
demo-model.txt and demo-result.txt, that your team leader will submit right away on EAS as Project 6.
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2 Evaluation Scheme
Students will be given individual grades that will be a function of the team grade and the peer-evaluation.
Team grade:
Code & Demo 40%
Report & Analysis 60%
Individual grade:
At the end of the project, all team members will fill-in a peer-evaluation form to evaluate the contribution
of each team member. The grade of a student will be a function of his/her team grade and the peer evaluation
received.
3 Submission
3.1 Code
The code is due Sunday April 7 at 6AM, 2019
Create one zip file, containing:
all your code
a README.txt file which will contain specific and complete instructions on how to run your program.
If the instructions in your readme file do not work or are incomplete, you will not be given the benefit
of the doubt.
the files baseline-model.txt, stopword-model.txt and wordlength-model.txt
the files stopword-result.txt, stopword-result.txt and wordlength-result.txt
the signed expectation of originality form (available on Moodle; or at: http://www.encs.concordia.
ca/documents/expectations.pdf)
Name your zip file: 472 P2 studentID1 studentID2 ….zip or 6721 P2 studentID1 studentID2 ….zip
where studentID1 is the team leader.
For example, 6721 P2 12345678.zip or 472 P2 12345678 87654321.zip
Have the Team Leader upload the zip file on EAS from his/her account as Project 5.
3.2 Demo
The demos will be during the lab periods on Monday April 8 and Thursday April 12, 2019.
During your demo, you will also submit the files demo-model.txt and demo-result.txt with the dataset
given by the TA. Submit these on EAS as Project 6.
3.3 Report
The report is due Friday April 12 at noon, 2019
Print your report and submit a paper copy in the appropriate assignment box in EV 3.177.
Follow the naming convention specified in Section 1.4, and have the Team Leader upload the PDF on EAS
from his/her account as Project 7.
Have fun!
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