One quick check to see whether a predictor is affecting your data in a non-PH way is to take the

following steps:

1. Split your data into two groups, based on the predictor.

2. Plot the Kaplan-Meier survival curve for each group (see Chapter 22).

If the two survival curves for a particular predictor display the slanted figure-eight pattern

shown in Figure 23-5, either don’t use PH regression on those data, or don’t use that predictor in

your PH regression model. That’s because it violates the assumption of proportional hazards

underlying PH regression.

© John Wiley & Sons, Inc.

FIGURE 23-5: Don’t try PH regression on this kind of data because it violates the PH assumption.

Your statistical software may offer several options to test the hazard-proportionality

assumption. Check your software’s documentation to see what it offers and how to interpret the

output. It may offer the following: