The P value tests the null hypothesis that the survival curves are identical in the overall populations. In this case, neither the log-rank nor the Wilcoxon-Gehan test rests will be very helpful. If two survival curves cross, then one group has a higher risk at early time points and the other group has a higher risk at late time points. Note that Prism cannot perform Cox proportional hazards regression. If in doubt, report the log-rank test (which is more standard) and report the Gehan-Wilcoxon results only if you have a strong reason. Ideally, this choice should be made before you collect and analyze your data. You need to choose which P value to report. The Gehan-Wilcoxon test does not require a consistent hazard ratio, but does require that one group consistently have a higher risk than the other.

In contrast, the log-rank test gives equal weight to all time points. This often makes lots of sense, but the results can be misleading when a large fraction of patients are censored at early time points.

The Gehan-Breslow-Wilcoxon method gives more weight to deaths at early time points.

Prism actually computes the Mantel-Haenszel method, which is nearly identical to the log-rank method (they differ only in how they deal with two subjects with the same time of death). One example of proportional hazards would be if the control group died at twice the rate as treated group at all time points. Proportional hazards means that the ratio of hazard functions (deaths per time) is the same at all time points.

The log-rank (Mantel-Cox) test is the more powerful of the two tests if the assumption of proportional hazards is true.

It doesn't ask for your preference, but always reports both. Prism compares two survival curves by two methods: the log-rank test (also called the Mantel-Cox test) and the Gehan-Breslow-Wilcoxon test. Interpreting results: Comparing two survival curves We could plot a survival curve for each cell line and compared them as follows: Data are censored when you simply don’t know what happened to the subject after that time, or do know but can’t use the

Enter ‘1’ for a subject if the event (death) occurred at the time entered into X.

Enter elapsed time (usually in days) as X (Prism does not let you enter dates).

(You can optionally use row titles to identify the subjects.) Each column represents a treatment. After, you enter data as follows:Įach row represents a distinct subject. With graphpad prism 5, you have to choose the survival button then you choose the kind of graph you want and the way to express your results (as fractions, percent.).

I used graphpad prism 5 to plot survival curves.

Graphpad prism 5 smooth curve