title: 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.
authors.affiliation: Department of Epidemiology, Channing Laboratory, Harvard School of Public Health, Brigham and Women's Hospital and Harvard Medical School, Boston, MA, USA. mstampfer@hsph.harvard.edu
authors.forename: Meir J
authors.initials: MJ
authors.lastname: Stampfer
mesh_terms: D001943:Breast Neoplasms / Q000209:etiology*; D002318:Cardiovascular Diseases / Q000209:etiology* / Q000517:prevention & control; D003627:Data Interpretation, Statistical; D015914:Estrogen Replacement Therapy / Q000009:adverse effects*; D005260:Female; D006306:Health Surveys; D006801:Humans; D008137:Longitudinal Studies; D008875:Middle Aged; D019370:Observation / Q000379:methods*; D016017:Odds Ratio; D016032:Randomized Controlled Trials as Topic / Q000379:methods*; D015983:Selection Bias; D013997:Time Factors