ITT for observational data: worst of both worlds?

Abstract

Hernán et al reanalyzed Nurses' Health Study Data on hormone therapy and heart disease, to explore further the apparent discrepancy for those results compared with findings from the Women's Health Initiative Trial. Hernán et al concludes that differences in time since menopause remains the most plausible explanation for the different findings. Part of the analysis employs application of the "intention-to treat" principle to analyze the observational data. This commentary points out some of the weaknesses inherent in that approach, which combines a major limitation of observational studies-lack of randomization-with a common limitation of trials, imperfect adherence to the assigned treatment.