Homework #4
Name:
_
Date:
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Dataset: The Stan Data Set, p271 of appendix 2. Some of the variables are also explained
on p118-9 of Chapter 5. The survival time used in this exercise will be from the date of
transplant surgery until death or censoring (i.e. surv2). All analyses will be restricted to
those patients who had transplants. An indicator variable called ‘trans’ is used to
determine whether a patient had a transplant (0 = No, 1 = Yes). The covariates to be
examined are surg, wtime, m3, and reject. Surg indicates whether the patient had prior
heart surgery. Wtime is the waiting time after acceptance into the study. M3 is a genetic
mismatch score. Reject is an indicator of rejection of the organ.
1.
Use proc lifetest to obtain a KM curve and a log survival plot.
2.
Use proc lifereg to determine whether any of the common models (exponential,
Weibull, Log-normal, and the standard gamma) are suitable for analysis when the
following covariates are included into the model: surg, wtime, m3, and reject. To
do this, carry out a likelihood ratio (LR) test to compare the common models with
the generalized gamma model (or use the routine provided at the end of the
lecture notes for chapter 4). Provide a table that gives the common models, their
associated Chi-Square statistics, the p-values, and your conclusion at the 5% level
of significance.
3.
(a) Regardless of the results in question 2, provide the analytical results of the
Weibull model.
(b) Are any of the covariates significant at the 5% level?
(c) What is your qualitative interpretation of the effect of the covariates on the
survival time at the 20% level of significance?
(d) What does the scale parameter tell us about the shape of the hazard function?
Hint: see bottom of page 68. Is this interpretation consistent with the logsurvival plot?
(e) Give a 90% C.I. for the parameter associated with the mismatch score.
(f) Estimate the hazard ratio for those people who rejected their organ to those
who did not. Hint: See pages 69-70.
4.
Provide a log-cumulative hazard plot for the stratification factor of organ
rejection. What does this plot tell us about the proportional-hazards assumption
for this particular case?