You're probably familiar with that Franciscan friar Sir William of Ockham, and his sacred saw. Apparently the principle has been as oversimplified as it has ignored, as a search of Wikipedia will attest. Suffice it to say, nonetheless, that this maxim guides us to select the simplest from among multiple explanations for any phenomenon - and this intuitively makes sense, because there are infinite and infinitely complex possible explanations for any phenomenon.
So I'm always amused and sometimes astonished when medical scientists reappraise their theories after they've been defeated by their very own data and begin to formulate increasingly complex explanations and apologies, so smitten and beholden to them as they are. "True Believers" is what Jon Abrams, MD, one of my former attendings, used to call them. The transition from scientist to theist is an insidious and subversive one.
The question is begged: did we design such and such clinical trial to test the null hypothesis or not? If some post-hoc subgroup is going to do better with therapy XYZ, why didn't we identify that a priori? Why didn't we test just THAT group? Why didn't we say, in advance, "if this trial fails to show efficacy, it will be because we should have limited it to this or that subgroup. And if it fails, we will follow up with a trial of this or that subgroup."