|Kaplan-Meier plot of survival for mice|
from Mitchell et al (2014)
So there goes my theory on why medical journals present median survival. Here we have a study where all the patients (they are mice) die, and yet the study still reports the (mostly) useless difference in median survival.
The study looked at the effect of the drug SRT1720 on life expectancy of mice and was recently reported in CELL. 400 mice were allocated to 4 treatment arms - standard diet, standard diet plus SRT1720, high-fat diet, and high-fat diet with SRT1720.
As I said, all the mice do the right thing and die and so we know the mean effect of SRT1720 on survival or we would if the authors had reported it. The authors do report that the average effect is "significant" for mice on both diets. I don't know if they mean it is statistically significant or medically significant. We also learn that SRT1720 is associated with an 8% increase in survival for the SD mice and a 22% increase in survival for the HD mice. We aren't told if these are statistically significantly different from zero.
While it is not discussed, we see from the picture that at about 85 weeks, the average increase in survival probability is twenty percentage points for the HD mice and ten percentage points for the SD mice. Although at 140 weeks, the average increase in the probability of survival due to the drug is approximately zero.
The fact that the curves come together at the end suggests that the drug affects different mice differently. Again this is not discussed by the authors, but we can infer from the figure that for at least 30% of mice on a high fat diet the drug increase survival (see discussion in this post). However, for some mice (that live a long time) the drug has no effect on survival.
As for diet, we see that has a very large effect. For the non-drugged mice, switching from HD to SD increases the probability of survival at 85 weeks about 40 percentage points. While at 140 weeks it increases the probability of survival by between 5 and 10 percentage points.
So if you are a mouse, you may want to cut down on the fat.