Calculating the time at which survival is 75% (or any percent)
Prism automatically computes and displays the median survival titme, the time at which survival is 50%. It does not compute the time at which survival is 75% or any other percentile. But you can easily do this yourself.
One of the pages in the survival results Prism creates as part of the survival analysis is a table of survival percentages.
In the example, for the control group, the survival at 46 days is 87.5% and the survival at time 78 days is 70.0%. The time of 75% survival has to be in between. Traditionally, survival curves are drawn as staircases, so the survival stays at 87.5% until the death at 78 days. So the 75th percentile would be defined as 78 days. Similarly, the 75th percentile for the treated group is 46 days. You can see these values on the graph belwo.
