Perhaps I misunderstand you, tada, but the point I was trying to make is that if ALL patients in a trial (even subconsciously) modify their lifestyle as a consequence of participating in it (remembering it involves repeated visits to a clinic, interviews about chest pain etc, blood draws, EKGs etc) the actual CVD incidence rate might be less than predicted in BOTH the treated and placebo groups.This reduces the statistical power of the study (meaning a bigger total sample size is needed to demonstrate an effect at a given P value). This is one possible reason for lower than expected event rates in trials.