Interpreting the P value

The P value tests the null hypothesis that the survival curves are identical in the overall populations. In other words, the null hypothesis is that the treatment did not change survival.

The P value answers this question:

If the null hypothesis is true, what is the probability of randomly selecting subjects whose survival curves are as different (or more so) than was actually observed?

Note that the P value is based on comparing entire survival curves, not on comparing only the median survival.

One-tail P value

Prism always reports a two-tail P value when comparing survival curves. If you wish to report a one-tail P value, you must have predicted which group would have the longer median survival before collecting any data. Computing the one-tail P value depends on whether your prediction was correct or not.

•If your prediction was correct, the one-tail P value is half the two-tail P value.

•If your prediction was wrong, the one-tail P value equals 1.0 minus half the two-tail P value. This value will be greater than 0.50, and you must conclude that the survival difference is not statistically significant.