# Inter-experimental variation in survival experiment - how to estimate variability?

In my animal experiments, I do survival studies, which generate Kaplan-Meier survival curves for each group, which I then compare with an appropriate log rank test.

My question is: If I have run a survival experiment with identical variables, say, five times, and the final outcome (in very layman's terms) happens to be slightly different each time, is there a test that I can do estimate the variability (variance) of my experimental runs?

One major confounding factor is the fact that survival is a continuous variable over time. Therefore, it is difficult to reduce it to a single statistic (that can then be compared with a test). A lot of people use the median survival (expressed in units of time) as a single statistic surrogate for survival, but it can often be misleading - depending upon the slope of the survival curve, and may not represent the true nature of the survival outcome.

Can someone here help? Please also let me know if further clarifications are needed.

• Can you provide an example of the outcomes you consider in your experiments? What is your sample size (in each group)?
– chl
Sep 30 '10 at 18:43
• I'm a bit confused what the goal is as well. Is there a reason examining a plot of each of the 5 hazard functions (and/or their confidence intervals) is insufficient? Do you need a test statistic to state where the curves intersect? As chl suggested in a comment to Thylacoleo's answer pooling seems inappropriate with different outcomes. Oct 1 '10 at 15:29
• Sorry about the late comment. I just received the book that Thylacoleo recommended. To answer your questions: Chl - the outcome is survival (i.e. no death) over a period of time. The sample size in each group varies from, say, 5-8. Andy - survival experiments suffer from inherent variabilities. I do indeed examine the Kaplan Meier plots of the groups by a log rank test. But I am more interested in finding out if there is a test that can evaluate inter-experimental variabilities of survival experiments. Nov 15 '10 at 20:47