Figure 1.

A genetic main effect, in a candidate gene study. The number of cases required to detect ORs from 1.1 to 3.0 for a genetic main effect with a power of 80% (at P < 0.0001—assuming a vague candidate gene) in a study with four controls per case. Assumptions (see Materials and methods section for details): (i) population genotypic prevalence = 9.75% [dashed lines] or 51% [solid lines], corresponding to dominant SNP effects with MAFs (minor allele frequencies) of 5 and 30%, respectively; (ii) genotypic ‘error’ corresponding to: R² = 1.0, 0.8 or 0.5; (iii) case-status determined with sensitivity 89.1% and specificity 97.4%: as for a study of diabetes diagnosed by a composite test (GP diagnosis or HbA1C ≥2 SD above the population mean⁵⁶); (iv) controls phenotypically assessed in the same way as cases; (v) incorporation of heterogeneity in the baseline risk of disease arising from unmeasured determinants, corresponding in magnitude to a 10-fold risk ratio between individuals on the high (95%) and low (5%) centiles of population risk