Applied Statistical Methods
HSRP 734
Summer 2008
Homework 6
Due: 7/31/2008
General instructions: 1) you may discuss any and all portions of the assignment with
other members of the class. However, the homework you turn in must be your own. 2)
For problems that require a statistical package, you can use any statistical software, but
must do your own programming. 3) A final answer is not sufficient. Show all your
work. Please provide relevant SAS output with clearly indicated answers to the
questions.
Q1. For a dataset with:
t = time to death
X1 = age – continuous variable
X2 = race: 0 - white, 1 - other
X3 = treatment group: 0 – standard drug, 1 – experimental drug
X4 = prognosis group: 0 – poor prognosis, 1 – fair prognosis, 2 – good prognosis
Q1a. State the form(i.e. formula) of the hazard function when fitting a Cox proportional
hazards model for time to death adjust for age, race, treatment group, and prognosis
group.
Q1b. State the form of the hazard function when fitting a stratified model that stratifies on
prognosis adjusted for age, race, and treatment group assuming no interaction between
prognosis and any predictors.
Q1c. State the form of the hazard function when fitting a stratified model that stratifies on
prognosis adjusted for age, race, and treatment group assuming an interaction between
prognosis and treatment group but no other interactions.
Q1d. For the model in c, what is the formula for the hazard ratio for the effect of
treatment group (experimental drug to standard) adjusted for the other variables?
Does this formula give a different hazard ratio for different prognosis groups? Why?
Q2. Use the SAS dataset hw5data from the class website to answer the following
questions. This dataset is the same as the one for homework 5
Q2a. Fit a stratified model that stratifies on diagnosis adjusted for M/F, age, bone marrow
transfer type, diagnosis and ferritin level assuming no interaction between diagnosis and
any predictors. What is the effect of ferritin level on the risk of death adjusted for the
other variables?
Q2b. Fit a stratified model that stratifies on diagnosis adjusted for M/F, age, bone marrow
transfer type, diagnosis and ferritin level assuming an interaction between diagnosis and
ferritin level but no other interactions. Is the effect of ferritin level different in the two
diagnosis groups? Why or why not?
Q2c. Whether the interaction term in the model from b was significant or not, use the
model in b to give the hazard ratio for a unit increase in ferritin level in AML patients and
the hazard ratio for a unit increase in ferritin level in patients with a diagnosis of ‘other’.