Overview
In this assignment you will write perceptron classifiers (vanilla and averaged) to identify hotel reviews as either truthful or deceptive, and either positive or negative. You may use the word tokens as features, or any other features you can devise from the text. The assignment will be graded based on the performance of your classifiers, that is how well they perform on unseen test data compared to the performance of a reference classifier.
Data
The training and development data are the same as for Coding Exercise 1. The uncompressed archive contains the following files:
• A top-level directory with two sub-directories, one for positive reviews and another for negative reviews (plus license and readme files which you won’t need for the exercise).
• Each of the subdirectories contains two sub-directories, one with truthful reviews and one with deceptive reviews.
• Each of these subdirectories contains four subdirectories, called “folds”.
• Each of the folds contains 80 text files with English text (one review per file).
The grading script will train your model on all of the training data, and test the model on unseen data in a similar format. The directory structure and file names of the test data will be masked so that they do not reveal the labels of the individual test files.
Programs
The perceptron algorithms appear in Hal Daumé III, A Course in Machine Learning (v. 0.99 draft), Chapter 4: The Perceptron.
You will write two programs: perceplearn.py will learn perceptron models (vanilla and averaged) from the training data, and percepclassify.py will use the models to classify new data. If using Python 3, you will name your programs perceplearn3.py and percepclassify3.py. You are encouraged to reuse your own code from Coding Exercise 1 for reading the data and writing the output, so that you can concentrate on implementing the classification algorithm.
The learning program will be invoked in the following way:
> python perceplearn.py /path/to/input
The argument is the directory of the training data; the program will learn perceptron models, and write the model parameters to two files: vanillamodel.txt for the vanilla perceptron, and averagedmodel.txt for the averaged perceptron. The format of the model files is up to you, but they should follow the following guidelines:
• The model files should contain sufficient information for percepclassify.py to successfully label new data.
• The model files should be human-readable, so that model parameters can be easily understood by visual inspection of the file.
The classification program will be invoked in the following way:
> python percepclassify.py /path/to/model /path/to/input
The first argument is the path to the model file (vanillamodel.txt or averagedmodel.txt), and the second argument is the path to the directory of the test data file; the program will read the parameters of a perceptron model from the model file, classify each entry in the test data, and write the results to a text file called percepoutput.txt in the following format:
label_a label_b path1
label_a label_b path2
⋮
In the above format, label_a is either “truthful” or “deceptive”, label_b is either “positive” or “negative”, and pathn is the path of the text file being classified.
Submission
All submissions will be completed through Vocareum; please consult the instructions for how to use Vocareum.
Multiple submissions are allowed; only the final submission will be graded. Each time you submit, a submission script is invoked. The submission script uses a specific portion of the training data as development data; it trains your model on the remaining training data, runs your classifier on the development data, and reports the results. Do not include the data in your submission: the submission script reads the data from a central directory, not from your personal directory. You should only upload your program files to Vocareum, that is percepclassify.py and perceplearn.py (plus any required auxiliary files, such as code shared between the programs or a word list that you wrote yourself).
You are encouraged to submit early and often in order to iron out any problems, especially issues with the format of the final output.
The performance of you classifier will be measured automatically; failure to format your output correctly may result in very low scores, which will not be changed.
For full credit, make sure to submit your assignment well before the deadline. The time of submission recorded by the system is the time used for determining late penalties. If your submission is received late, whatever the reason (including equipment failure and network latencies or outages), it will incur a late penalty.
Grading
After the due date, we will train your model on the full training data (including development data), run your classifier on unseen test data twice (once with the vanilla model, and once with the averaged model), and compute the F1 score of your output for each of the four classes (truthful, deceptive, positive, and negative). Your grade will be based on the performance of your classifier. We will calculate the mean of the four F1 scores for each model and scale it to the performance of a perceptron classifier developed by the instructional staff (so if that classifier has F1=0.8, then a score of 0.8 will receive a full credit, and a score of 0.72 will receive 90% credit; your vanilla perceptron will be compared to a reference vanilla perceptron, and your averaged perceptron will be compared to a reference averaged perceptron). The overall grade will be the mean of the grades for the vanilla and averaged perceptrons.
Notes
• Development data. While developing your programs, you should reserve some of the data as development data in order to test the performance of your programs. The submission script on Vocareum will use folds 2, 3, and 4 as training data, and fold 1 as development data: that is, it will run perceplearn.py on a directory containing only folds 2, 3, and 4, and it will run percepclassify.py on a directory with a modified version of fold 1, where directory and file names are masked. While developing on your own you may use different splits of the data (but to get the same results as the submission script, you’ll need to use the same split). The grading script will use all 4 folds for training, and unseen data for testing.
• Problem formulation. Since a perceptron is a binary classifier, you need to treat the problem as two separate binary classification problems (truthful/deceptive and positive/negative); each of the model files (vanilla and averaged) needs to have the model parameters for both classifiers.
• Features and tokenization. You’d need to develop some reasonable method of identifying features in the text. Some common options are removing certain punctuation, or lowercasing all the letters. You may also find it useful to ignore certain high-frequency or low-frequency tokens. You may use any tokenization method which you implement yourselves. Experiment, and choose whichever works best.
• Runtime efficiency. Vocareum imposes a limit on running times, and if a program takes too long, Vocareum will kill the process. Your program therefore needs to run efficiently. You need an efficient way to store the training instances, in order to avoid reading them over and over again (reading and parsing text is slow). Also, feature vectors for individual training instances are typically fairly sparse: for a reference solution with about 1000 features, the mean number of non-zero features per training instance is about 77; it would be highly inefficient to multiply and add all the 900+ zeros at every step. The reference solution stores the training data as a python dict indexed by the unique identifiers of the reviews, and the feature vector for each training instance as a dict of the form feature:count. With about 1000 features and 100 iterations (which is probably more than needed, due to overfitting), run times for the reference solution are under 5 seconds for running perceplearn.py on the training data, running on a MacBook Pro from 2016.
• Overfitting. The perceptron has a tendency to overfit the training data, so you should experiment in order to find out a good number of iterations to stop at.
• You may not look for solutions on the web, or use code you find online or anywhere else.
• You may not download the data from any source other than the files provided on Blackboard, and you may not attempt to locate the test data on the web or anywhere else.
• You may use packages in the Python Standard Library, as well as numpy. You may not use any other packages.
• You may use external resources to learn basic functions of Python (such as reading and writing files, handling text strings, and basic math), but the extraction and computation of model parameters, as well as the use of these parameters for classification, must be your own work.
• We use plagiarism detection software to identify similarities between student assignments, and between student assignments and known solutions on the web. Any attempt to fool plagiarism detection, for example the modification of code to reduce its similarity to the source, will result in an automatic failing grade for the course.