SAMPLE-SIZE CALCULATIONS FOR STUDIES WITH CORRELATED OBSERVATIONS

Correlated data occur frequently in biomedical research. Examples incl ude longitudinal studies, family studies, and ophthalmologic studies. In this paper, we present a method to compute sample sizes and statist ical powers for studies involving correlated observations. This is a m ultivariate extension of the work by Self and Mauritsen (1988, Biometr ics 44, 79-86), who derived a sample size and power formula for genera lized linear models based on the score statistic. For correlated data, we appeal to a statistic based on the generalized estimating equation method (Liang and Zeger, 1986, Biometrika 73, 13-22). We highlight th e additional assumptions needed to deal with correlated data. Some spe cial cases that are commonly seen in practice are discussed, followed by simulation studies.