Generalizability vs. Transportability in Trials

Suhail- I’m not disputing the practice of combining an individual patient’s estimated baseline risk for an event with an RCT-derived OR in order to estimate the personal absolute risk reduction he might expect with therapy. Rather, I’m disputing the idea that a group -level ARR, as derived from an RCT , can be “translated/transported” (or whatever the right word is) to an individual patient in the postmarket setting to estimate his personal ARR.

Within-arm event rates will differ between trials, as will the between-arm difference in event rates, even when the trials are designed identically. Therefore, the NNT calculated from these identically-designed trials plausibly might end up differing substantially (!)

I’m highlighting the fact that the ARR derived from an RCT is a group -level assessment of absolute risk reduction that should NOT be conflated with the absolute risk reduction that an individual patient in the postmarket setting might incur. An RCT-derived ARR reflects the covariate distribution of its underlying convenience sample and this distribution can change from trial to trial. Therefore, “NNT” is highly situational and dependent on features of the recruited convenience sample. As such, NNT should NOT be “transported” to individual patients in the postmarket setting for the purpose of therapeutic decision-making (?)