Ratio of median survival times

If you compare two survival curves, Prism reports the ratio of the median survival times along with its 95% confidence interval of the ratio.

This calculation of the confidence interval of the ratio of survival times is based on an assumption that is not part of the rest of the survival comparison:  that both survival curves follow an exponential decay. This means that the chance of dying in a small time interval is the same early in the study and late in the study. If your survival data follow a very different pattern, then the values that Prism reports for the 95% CI of the ratio of median survivals will not be meaningful.

Note that prior versions of Prism computed the confidence interval incorrectly (but computed the ratio just fine).

Why Prism doesn't compute the confidence interval of median survival time

While Prism computes the confidence interval for the ratio of median survivals (when you compare two groups), it does not compute the 95% confidence interval for the median survival time itself. The reason is that multiple methods for computing a confidence interval of median survival have been published and none seem to be standard, and the results don't match. To read more:

•One method is in Collett starting at page 35 .

•Brookmeer and Crowley, A confidence interval for the median survival time. Biometrics (1982) vol. 38 (1) pp. 29-41.

•Barker reviews several methods and points out how different their results can be. The Mean, Median, and Confidence Intervals of the Kaplan-Meier Survival Estimate—Computations and Applications. The American Statistician (2009) vol. 63 (1) pp. 78-80