The objectives of this paper are to (1) examine methods of using longitudinal data in designing comparative trials and calculating sample sizes or power and (2) show the effect of autocorrelation of repeated measures on the assessment of sample sizes. A statistical model with a simple regression structure for the mean trajectory of the longitudinal data and a two-parameter model for the correlations of within-individual observations given by corr(yt,yt+s) = γsθ is used. The methods are illustrated by considering a two-group trial and investigating the effect of different values of the correlation parameters, γ and θ on the sample size. The results show that taking account of the autocorrelation structure of longitudinal data may lead to more efficient designs. Specifically, the stronger the autocorrelation is, the smaller the sample size that is required.
Bibliographical noteFunding Information:
Received from the Department of Epidemiology, the Johns Hopkins School of Hygiene and Public Health, Baltimore, Maryland. Accepted for publication September 24, 1993. This work was supported by contract NO1-AI-72676 from the National Institute of Allergy and Infectious Disease. This paper was selected for presentation at the Student Scholarship session of the 14th Annual Meeting of the Society of Clinical Trials. Address correspondence to Dr. A. Kirby, Hampton House, Rm. 782, 624 North Broadway, Baltimore, MD 21205.
- biological markers
- longitudinal studies
- sample size
ASJC Scopus subject areas