If cases are sampled from all those present in the general population, the relative risks (ODDS RATIOS in a case-control study) for specific alleles influencing complex diseases are expected to be modest to small 31. For polymorphisms with allele frequencies >0.2, the odds ratios are expected be in the range of 1.1-1.5; for allele frequencies between 0.05 and 0.2, up to approximately 3.0. This is true by definition, since a common variant with much larger relative risks would result in a large attributable risk for that variant with respect to the disease; in other words, the variant would explain a very large proportion of the causality of the disease, which would make the condition’s characteristics resemble a Mendelian rather than a complex disorder. As a guideline, sample sizes of at least 1,000 cases and 1,000 controls are required to detect odds ratios around 1.5 in size with at least 80% power, but the required size of each individual study will depend on whether 1) the analysis will also include case sub-groups; 2) the analysis focuses on candidate genes with a
