prevalence 0.1%, with the number of control samples ranging from 500 to 5,000 individuals at a significance level of 5%. Power is presented as a function of the heterozygous genotype relative risk under the assumption of a multiplicative disease model. Improvements in power diminish as the ratio of controls to cases increases. When the number of available cases is the limiting factor, a control-case ratio of 4:1 is often cited as optimal, but this is a judgement and the exact rate of diminishing return may vary according to disease risk and allele frequency, as well as analytical issues, such as investigation of interactions or subgroup specific effects. However, if additional genotypes are effectively free, power can be maximized by including as many controls as possible in the study.
