Could you elaborate on this? Does this assume "you" extrapolate the effect of treatment A to the general population?

I mean, I understand that if you have a sample size of two, find that treatment A does not induce an effect, and conclude that treatment A has no effect [for all test subjects] this does not hold. However, surely if the sample size is big enough (which obviously isn't always clear, but for the sake of the argument let's assume it is) then drawing such conclusions does hold (within the certainty thresholds predefined for your statistical test of choice, such as 99% or 95% probability). Or have I misunderstood?