COHORT VERSUS CROSS-SECTIONAL DESIGN IN LARGE FIELD TRIALS - PRECISION, SAMPLE-SIZE, AND A UNIFYING MODEL

In planning large longitudinal field trials, one is often faced with a choice between a cohort design and a cross-sectional design, with att endant issues of precision, sample size, and bias. To provide a practi cal method for assessing these trade-offs quantitatively, we present a unifying statistical model that embraces both designs as special case s. The model takes account of continuous and discrete endpoints, site differences, and random cluster and subject effects of both a time-inv ariant and a time-varying nature. We provide a comprehensive design eq uation, relating sample size to precision for cohort and cross-section al designs, and show that the follow-up cost and selection bias attend ing a cohort design may outweigh any theoretical advantage in precisio n. We provide formulae for the minimum number of clusters and subjects . We relate this model to the recently published prevalence model for COMMIT, a multi-site trial of smoking cessation programmes. Finally, w e tabulate parameter estimates for some physiological endpoints from r ecent community-based heart-disease prevention trials, work an example , and discuss the need for compiling such estimates as a basis for inf ormed design of future field trials.