I guess my question would have been why was the trial not designed to be sufficiently powered to do this analysis?  As far as an a priori hypothesis and prior evidence justifying the sub group analysis, how about the study in the NEJM which compared rosuvastatin and atorvastatin. It found that rosuvastatin was more effective at both lowering LDL and raising HDL so it would seem the drugs perhaps act differently somehow?

Not sufficiently powered?  I assume it was a decision based on N of subjects required and $$$ they didn't have.  It's partly that.  Also what Bear and I said yesterday.  You have to have prior evidence and a theory to set up a priori subgroup analyses.

ASSURE primary outcome was looking at plaque regression.  If this is the NEJM study you're referring to....

"....Despite the lower level of LDL cholesterol and the higher level of HDL cholesterol achieved with rosuvastatin, a similar degree of regression of PAV was observed in the two treatment groups...."

(my bolding)

https://www.nejm.org/doi/pdf/10.1056/NEJMoa1110874