Calculating lifespan extension as additional years of life after midlife is a less informative statistic for comparing drugs or labs than median lifespan extension.

“it's not i think terribly informative there's an easy to imagine thought experiment you've got someone on a ventilator he or she is on death's doorstep they're gonna die today and you leave them on the ventilator for one more day and then you give up and they die so you've extended you've doubled their lifespan right you've gone from one day to two days that's a hundred percent increase in lifespan so when i put it that way you can see that that's not a very useful statistic to calculate”