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Assignment 2 (DDL: 2024/10/20)

1. Point Estimation (15 pts)
The Poisson distribution is a useful discrete distribution which can be used to model the
number of occurrences of something per unit time. For example, in networking, packet arrival
density is often modeled with the Poisson distribution. If is Poisson distributed, i.e., its probability mass function takes the following form:

2. Source of Error: Part 1 (15 pts)
Suppose that we are given an independent and identically distributed sample of points { &}
where each point & ∼ ( , 1) is distributed according to a normal distribution with mean
and variance 1. You are going to analyze different estimators of the mean .
(a) Suppose that we use the estimator ̂= 1 for the mean of the sample, ignoring the
observed data when making our estimate. Give the bias and variance of this estimator ̂.
Explainin a sentence whether this is a good estimator in general, and give an example of
when this is a good estimator.
(b) Now suppose that we use ̂= $ as an estimator of the mean. That is, we use the first
data point in our sample to estimate the mean of the sample. Give the bias and variance
of thisestimator ̂. Explain in a sentence or two whether this is a good estimator or not.
(c) In the class you have seen the relationship between the MLE estimator and the least
squares problem. Sometimes it is useful to use the following estimate
&'$

For the mean, where the parameter > 0 is a known number. The estimator ̂ is biased,
but has lower variance than the sample mean ̅= "$ ∑& & which is an unbiased
estimator for . Give the bias and variance of the estimator ̂.

3. Source of Error: Part 2 (15 pts)
In class we discussed the fact that machine learning algorithms for function approximation
are also a kind of estimator (of the unknown target function), and that errors in function
approximation arise from three sources: bias, variance, and unavoidable error. In this part of
the question you are going to analyze error when training Bayesian classifiers. Suppose that is boolean, is real valued, ( = 1) = 1/2 and that the class conditional
distributions ( | ) are uniform distributions with ( | = 1) = [1,4] and
( | = 0) = [−4, −1]. (we use [ , ] to denote a uniform probability
distribution between and , with zero probability outside the interval [ , ]).
(a) Plot the two class conditional probability distributions ( | = 0) and ( | = 1).
(b) What is the error of the optimal classifier? Note that the optimal classifier knows ( =
1) , ( | = 0) and ( | = 1) perfectly, and applies Bayes rule to classify new
examples. Recall that the error of a classifier is the probability that it will misclassify a new
drawn at random from ( ). The error of this optimal Bayes classifier is the unavoidable
error for this learning task.
(c) Suppose instead that ( = 1) = 1/2 and that the class conditional distributions are
uniform distribution with ( | = 1) = [0,4] and ( | = 0) =
[−3,1]. What isthe unavoidable error in this case? Justify your answer.
(d) Consider again the learning task from part (a) above. Suppose we train a Gaussian Naive
Bayes (GNB) classifier using training examples for this task, where → ∞. Of course our
classifier will now (incorrectly) model ( | ) as a Gaussian distribution, so it will be
biased: it cannot even represent the correct form of ( | ) or ( | ).
Draw again the plot you created in part (a), and add to it a sketch of the learned/estimated
class conditional probability distributions the classifier will derive from the infinite training
data. Write down an expression for the error of the GNB. (hint: your expression will
involve integrals - please don't bother solving them).
(e) So far we have assumed infinite training data, so the only two sources of error are bias
and unavoidable error. Explain in one sentences how your answer to part (d) above would
change if the number of training examples was finite. Will the error increase or decrease?
Which of the three possible sources of error would be present in this situation?

4. Gaussian (Naïve) Bayes and Logistic Regression (15 pts)
Recall that a generative classifier estimates ( , ) = ( ) ( | ), while a discriminative
classifier directly estimates ( | ). (Note that certain discriminative classifiers are nonprobabilistic:
they directly estimate a function ∶ → instead of ( | ).) For clarity, we
highlight in bold to emphasize that it usually represents a vector of multiple attributes, i.e.,
= { $, +, . . . , %}. However, this question does not require students to derivethe answer
in vector/matrix notation.
In class we have observed an interesting relationship between a discriminative classifier
(logistic regression) and a generative classifier (Gaussian naive Bayes): the form of
( | ) derived from the assumptions of a specific class of Gaussian naive Bayes classifiers is
precisely the form used by logistic regression. The derivation can be found in the required
reading: http://www.cs.cmu.edu/~tom/mlbook/NBayesLogReg.pdf.We made the following
assumptions for Gaussian naive Bayes classifiers to model ( , ) = ( ) ( | ):
(1) is a boolean variable following a Bernouli distribution, with parameter = ( = 1)
and thus ( = 0) = 1 − .
(2) = { $, +, . . . , %}, where each attribute & is a continuous random variable. For each
& , ( &| = ) is a Gaussian distribution ( &,, &) . Note that & is the standard
deviation of the Gaussian distribution (and thus &
+ is the variance), which does not
depend on .
(3) For all ≠ , & and - are conditionally independent given . This is why this type of
classifier is called “naive”. We say this is a specific class of Gaussian naive Bayes classifiers because we have made an
assumption that the standard deviation & of ( &| = ) does not depend on the value of
. This is not a general assumption for Gaussian naive Bayes classifiers.
Let's make our Gaussian naive Bayes classifiers a little more general by removing the
assumption that the standard deviation & of ( &| = ) does not depend on . As a result,
for each &, ( &| = ) is Gaussian distribution ( &,, &,), where = 1,2, . . . , and =
0,1. Note that now the standard deviation &, of ( &| = ) depends on both the attribute
index and the value of .
Question: is the new form of ( | ) implied by this more general Gaussian naive Bayes
classifier still the form used by logistic regression? Derive the new form of ( | ) to prove
your answer.

5. Programming (40 pts)
In this lab, please submit your code according to the following guidelines:
(a) Cross-Validation: https://qffc.uic.edu.cn/home/content/index/pid/276/cid/6530.html
Please try these three approaches holdout, K-fold and leave-p-out with the data file 2.1-
Exercise.csv.
Submit ‘Exercise-handout.py’, ‘Exercise-k-fold.py’, and ‘Exercise-leave-p-out.py’
(b) Linear regression: https://qffc.uic.edu.cn/home/content/index/pid/276/cid/6541.html
Please modify linear_regression_lobf.py with the data file 2.2-Exercise.csv. For this task,
take the High column values as variables and Target column for prediction.
Submit ‘Exercise-linear_regression_lobf.py’
(c) Naïve Bayes: https://qffc.uic.edu.cn/home/content/index/pid/276/cid/6557.html
Here the dataset ‘basketball.csv’ used is for basketball games and weather conditions
where the target is if a basketball game is played in the given conditions or not, the
dataset is very small, just containing 14 rows and 5 columns.
Submit ‘Exercise-NB.py’
(d) Logistic regression: https://qffc.uic.edu.cn/home/content/index/pid/276/cid/6556.html
Use breast cancer from sklearn using following code: from sklearn.datasets import
load_breast_cancer.
Submit ‘Exercise-Logistic-Regression.py’