Median survival

Medians are used rather than averages because some patients live very long and the average would be dragged up and undefined until the last patient dies; the median can be read off the survival curve at the point where it crosses 50%. Comparing medians between arms gives an absolute benefit in months, complementing the hazard ratio's relative measure, but it captures only one point on the curve: two treatments with the same median can differ hugely in the proportion alive at three years, which is where immunotherapy's long tail shows up. 'Median not reached' means fewer than half the patients had the event at the time of analysis, a good sign that is also frustratingly uninformative.