Observational versus randomized trial evidence (Correspondence)

Authors' reply When observational epidemiological data and randomised controlled trials produce contradictory findings there is a need to explore the reasons. We showed that the relative risks of mortality from coronary heart disease (CHD) for an identical difference in plasma vitamin C (15·7 μmol/L) suggested a strong beneficial effect in the EPIC observational study1 and weak evidence of possible harm in the Heart Protection randomised controlled trial.2 Heart Protection Study Collaborative Group , MRC/BHF Heart Protection Study of antioxidant vitamin supplementation in 20536 high-risk individuals: a randomised placebo-controlled trial, Lancet 360 (2002), pp. 23–33.2 These differences seem to us to be contradictory rather than consistent. If the observational findings were always clearly explained by factors for ....