Proportional hazards

The Cox regression assumes that the ratio of the hazards of two groups stays the same over time. Statable checks this with the score test cox.zph. p below .05 speaks against the assumption: the HR then changes over time.

Example: p = .012 for the group: the HR is not constant; a single HR summarizes it only roughly.

Caution: With many people the test flags even small departures, with few it misses large ones. Crossing curves in the chart are a clear sign.