Biostatistics 515, Winter 2004
Biostatistics 515, Winter 2004
Homework 8 solutions/comments
2. WHAS data: there are total 481 subjects in the data.
grouped cohort alive=0 dead=1 total follow-up time(min, max)
1 58 102 160 (1, 5843)
2 82 93 175 (1, 3665)
3 92 54 146 (1, 1829)
a) The size of grouped cohort 1 is n=160. Fitted the Cox PH model is the following:
coxph(formula = Surv(lenfol, fstat) ~ age + sex + cpk + chf +
miord, data = whas, subset = (yrgrp == 1))
coef exp(coef) se(coef) z p
age 3.41e-02 1.03 0.009629 3.5410 0.0004
sex 1.26e-02 1.01 0.211653 0.0598 0.9500
cpk -2.38e-05 1.00 0.000174 -0.1370 0.8900
chf 3.80e-01 1.46 0.212086 1.7909 0.0730
miord 1.93e-01 1.21 0.203880 0.9463 0.3400
exp(coef) exp(-coef) lower .95 upper .95
age 1.03 0.966 1.015 1.05
sex 1.01 0.987 0.669 1.53
cpk 1.00 1.000 1.000 1.00
chf 1.46 0.684 0.965 2.22
miord 1.21 0.825 0.813 1.81
(b) Model 1: [pic]
H0: βi = 0 , for i = 1 … 5
H1: at least one of βi is not equal to 0
LR = -2logL(reduced model) –[-2logL(full model)] = 25.4 > 11.07= [pic]
So, we reject the null hypothesis at α = 0.05 significant level and conclude that the model is significant.
(c)There are five parameters in the above model, so we will perform five Wald tests to see if each covariate contribute to the model given other covariates are in the model.
---- Testing sex given age, cpk, chf and miord in the model:
H0: β2 = 0
H1: β2 ≠ 0
Z score is 0.0598 ................
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