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# STAT 435 Homework # 2 solution

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STAT 435
Homework # 2
Online Submission Via Canvas
Instructions: You may discuss the homework problems in small groups, but you
must write up the final solutions and code yourself. Please turn in your code for the
problems that involve coding. However, for the problems that involve coding, you
must also provide written answers: you will receive no credit if you submit code without written answers. You might want to use Rmarkdown to prepare your assignment.
1. Suppose we have a quantitative response Y , and a single feature X ∈ R. Let
RSS1 denote the residual sum of squares that results from fitting the model
Y = β0 + β1X + ?

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STAT 435
Homework # 2
Online Submission Via Canvas
Instructions: You may discuss the homework problems in small groups, but you
must write up the final solutions and code yourself. Please turn in your code for the
problems that involve coding. However, for the problems that involve coding, you
must also provide written answers: you will receive no credit if you submit code without written answers. You might want to use Rmarkdown to prepare your assignment.
1. Suppose we have a quantitative response Y , and a single feature X ∈ R. Let
RSS1 denote the residual sum of squares that results from fitting the model
Y = β0 + β1X + ?
using least squares. Let RSS12 denote the residual sum of squares that results
from fitting the model
Y = β0 + β1X + β2X
2 + ?
using least squares.
(b) Prove that the R2 of the model containing just the feature X is no greater
than the R2 of the model containing both X and X2
.
2. Describe the null hypotheses to which the p-values in Table 3.4 of the textbook correspond. Explain what conclusions you can draw based on these pvalues. Your explanation should be phrased in terms of sales, TV, radio, and
newspaper, rather than in terms of the coefficients of the linear model.
3. Consider a linear model with just one feature,
Y = β0 + β1X + ?.
Suppose we have n observations from this model, (x1, y1), . . . ,(xn, yn). The
least squares estimator is given in (3.4) of the textbook. Furthermore, we saw
1
in class that if we construct a n × 2 matrix X˜ whose first column is a vector of
1’s and whose second column is a vector with elements x1, . . . , xn, and if we let
y denote the vector with elements y1, . . . , yn, then the least squares estimator
takes the form
?
βˆ
0
βˆ
1
?
=
?
X˜ TX˜
?−1
X˜ T y. (1)
Prove that (1) agrees with equation (3.4) of the textbook, i.e. βˆ
0 and βˆ
1 in (1)
equal βˆ
0 and βˆ
1 in (3.4).
4. This question involves the use of multiple linear regression on the Auto data
set, which is available as part of the ISLR library.
(a) Use the lm() function to perform a multiple linear regression with mpg as
the response and all other variables except name as the predictors. Use
the summary() function to print the results. Comment on the output. For
instance:
i. Is there a relationship between the predictors and the response?
ii. Which predictors appear to have a statistically significant relationship
to the response?
iii. Provide an interpretation for the coefficient associated with the variable year.
Make sure that you treat the qualitative variable origin appropriately.
(b) Try out some models to predict mpg using functions of the variable horsepower.
Comment on the best model you obtain. Make a plot with horsepower
on the x-axis and mpg on the y-axis that displays both the observations
and the fitted function (i.e. ˆf(horsepower)).
(c) Now fit a model to predict mpg using horsepower, origin, and an interaction between horsepower and origin. Make sure to treat the qualitative
variable origin appropriately. Comment on your results. Provide a careful interpretation of each regression coefficient.
5. Consider fitting a model to predict credit card balance using income and
student, where student is a qualitative variable that takes on one of three
(a) Encode the student variable using two dummy variables, one of which
equals 1 if student=graduate (and 0 otherwise), and one of which equals
1 if student=undergraduate (and 0 otherwise). Write out an expression
for a linear model to predict balance using income and student, using
this coding of the dummy variables. Interpret the coefficients in this linear
model.
(b) Now encode the student variable using two dummy variables, one of which
equals 1 if student=not student (and 0 otherwise), and one of which
2
equals 1 if student=graduate (and 0 otherwise). Write out an expression
for a linear model to predict balance using income and student, using
this coding of the dummy variables. Interpret the coefficients in this linear
model.
(c) Using the coding in (a), write out an expression for a linear model to predict balance using income, student, and an interaction between income
and student. Interpret the coefficients in this model.
(d) Using the coding in (b), write out an expression for a linear model to predict balance using income, student, and an interaction between income
and student. Interpret the coefficients in this model.
(e) Using simulated data for balance, income, and student, show that the
fitted values (predictions) from the models in (a)–(d) do not depend on
the coding of the dummy variables (i.e. the models in (a) and (b) yield
the same fitted values, as do the models in (c) and (d)).
6. Extra Credit. Consider a linear model with just one feature,
Y = β0 + β1X + ?,
with E(?) = 0 and Var(?) = σ
2
. Suppose we have n observations from this
model, (x1, y1), . . . ,(xn, yn). We assume that x1, . . . , xn are fixed, so the only
randomness in the model comes from ?1, . . . , ?n. Use (3.4) in the textbook
— or, if you prefer, the matrix algebra formulation in (1) of this homework
assignment — in order to derive the expressions for Var(βˆ
0) and Var(βˆ
1) given
in (3.8) of the textbook.
3