The commonly used log-rank test for comparing survival times between groups of patients is a special case of the test of association between the grouping variable and survival time in a Cox proportional hazards regression model. Depending on how one handles tied failure times, the log-rank

χ2 statistic exactly equals the score χ2 statistic from the Cox model, and the likelihood ratio and Wald χ2 test statistics are also appropriate. To obtain global score or LR χ2 tests and P-values you can use a statement as the following, where cph is in the rms package. It is similar to the survival package’s coxph function.

cph(Survobject ∼ predictor)

Here Survobject is a survival time object created by the Surv function. Obtain the log-rank (score) χ2 statistic, degrees of freedom, and P-value for testing for differences in survival time between levels of dzclass. Interpret this test, referring to the graph you produced in Problem 6 if needed.