# Assignment 4 CS 750/850 Machine Learning solved

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## Description

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Problem 1 [25%]
In this exercise, we will predict the number of applications received using the other variables in the College
(ISLR::College) data set.
1. Fit a linear model using least squares on the training set, and report the test error obtained.
2. Use best subset selection with cross-validation. Report the test error obtained.
3. Fit a ridge regression model on the training set, with λ chosen by cross-validation.
4. Fit a lasso model on the training set, with λ chosen by cross-validation. Report the test error obtained,
along with the number of non-zero coefficient estimates.
5. Briefly comment on the results obtained. How accurately can we predict the number of college
applications received? Is there much difference among the test errors resulting from these approaches?
Problem 2 [25%]
We will try to predict per capita crime rate in the Boston dataset.
1. Try out best subset selection, the lasso, ridge regression, and PCR on this problem. Present and discuss
results for the approaches that you consider.
2. Propose a model (or set of models) that seem to perform well on this data set, and justify your answer.
Make sure that you are evaluating model performance using validation set error, cross-validation, or
some other reasonable alternative, as opposed to using training error.
Problem 3 [25%]
Suppose we have a linear regression problem with P features. We estimate the coefficients in the linear
regression model by minimizing the RSS for the first p features:
Xn
i=1

yi − β0 −
Xp
j=1
βjxij

2
where p ≤ P. For parts (1) through (5), indicate which of i. through v. is correct. Briefly justify your
1. As we increase p from 1 to P, the training RSS will typically:
i. Remain constant.
iv. Increase initially, and then eventually start decreasing in an inverted U shape.
v. Decrease initially, and then eventually start increasing in a U shape.
1
2. Repeat (1) for test MSE.
3. Repeat (1) for squared bias.
4. Repeat (1) for variance.
5. Repeat (1) for the irreducible error (Bayes error).
Problem 4 [25%]
Suppose we estimate the regression coefficients in a linear regression model by minimizing
Xn
i=1

yi − β0 −
Xp
j=1
βjxij

2
subject to Xp
j=1
|βj | ≤ s
for a particular value of s. For parts (1) through (5), indicate which of i. through v. is correct. Justify your
1. As we increase s from 0, the training RSS will typically:
i. Remain constant.