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# Assignment 2-Linear Classiﬁcation

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Linear Classiﬁcation and Nearest Neighbor Classiﬁcation
1. You will use a synthetic data set for the classiﬁcation task that you’ll generate yourself. Generate two classes with 20 features each. Each class is given by a multivariate Gaussian distribution, with both classes sharing the same covariance matrix. You are provided with the mean vectors (DS1-m0 for mean vector of negative class and DS1-m1 for mean vector of positive class) and the covariance matrix (DS1-cov). Generate 2000 examples for each class, and label the data to be positive if they came from the Gaussian with mean m1 and negative if they came from the Gaussian with mean m0. Randomly pick 30% of each class (i.e., 600 data points per class) as a test set, and train the classiﬁers on the remaining 70%. data When you report performance results, it should be on the left out 30%. Call this dataset at DS1, and submit it with your code.
2. We ﬁrst consider the probabilistic LDA model as seen in class: given the class variable, the data are assumed to be Gaussians with diﬀerent means for diﬀerent classes but with
the same covariance matrix. This model can formally be speciﬁed as follows: Y ∼ Bernoulli(π), X|Y = j ∼ N(µj,Σ).
Estimate the parameters of the probabilistic LDA model using the maximum likelihood approach. For DS1, report the best ﬁt accuracy, precision, recall and F-measure achieved by the classiﬁer, along with the coeﬃcients learnt.
3. For DS1, use k-NN to learn a classiﬁer. Repeat the experiment for diﬀerent values of k and report the performance for each value. We will compare this non-linear classiﬁer to the linear approach, and ﬁnd out how powerful linear classiﬁers can be. Do you do better than LDA or worse? Are there particular values of k which perform better? Report the best ﬁt accuracy, precision, recall and f-measure achieved by this classiﬁer.
4. Now instead of having a single multivariate Gaussian distribution per class, each class is going to be generated by a mixture of 3 Gaussians. For each class, we’ll deﬁne 3 Gaussians, with the ﬁrst Gaussian of the ﬁrst class sharing the covariance matrix with the ﬁrst Gaussian of the second class and so on. For both the classes, ﬁx the mixture probability as (0.1,0.42,0.48) i.e. the sample has arisen from ﬁrst Gaussian with probability 0.1, second with probability 0.42 and so on. Mean for three Gaussians in the positive class are given as DS2-c1-m1, DS2-c1-m2, DS2-c1-m3. Mean for three Gaussians in the negative class are gives as DS2-c2-m1, DS2-c2-m2, DS2-c2-m3. Corresponding 3 covariance matrices are given as DS2-cov-1, DS2-cov-2 and DS2-cov-3. Now sample from this distribution and generate the dataset similar to question 1. Call this dataset as DS2, and submit it with your code.
5. Now perform the experiments in questions 2 and 3 again, but now using DS2. Report the same performance measures as before. What do you observe?
6. Comment on any similarities and diﬀerences between the performance of both classiﬁers on datasets DS1 and DS2?
Instruction for code submission
1. Submit a single zipped folder with your McGill id as the name of the folder. For example if your McGill ID is 12345678, then the submission should be 12345678.zip.
2. If you are using python, you must submit your solution as a jupyter notebook.
3. Make sure all the data ﬁles needed to run your code is within the folder and loaded with relative path. We should be able to run your code without making any modiﬁcations.
Instruction for report submission
1. You report should be brief and to the point. When asked for comments, your comment should not be more than 3-4 lines.
Page 2
2. Do not include your code in the report!
3. If you report consists of more than one page, make sure the pages are stapled.

Assignment 2-Linear Classiﬁcation
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