Adjustments for center in multicenter studies: An overview

Increasingly, investigators rely on multicenter or multigroup studies to de monstrate effectiveness and generalizability. Authors too often overlook th e analytic challenges in these study designs: the correlation of outcomes a nd exposures among patients within centers, confounding of associations by center, and effect modification of treatment or exposure across center. Cor relation or clustering, resulting from the similarity of outcomes among pat ients within a center, requires an adjustment to confidence intervals and P values, especially in observational studies and in randomized multicenter studies in which treatment is allocated by center rather than by individual patient. Multicenter designs also warrant testing and adjustment for the p otential bias of confounding by center, and for the presence of effect modi fication or interaction by center. This paper uses examples from the recent biomedical literature to highlight the issues and analytic options.