One consequence of using a shared control group (for which detailed phenotyping for all traits of interest is not available) relates to the potential for misclassification bias: a proportion of the controls is likely to have the disease of interest (and therefore might meet the criteria for inclusion as a case) and some others will develop it in the future. However, the effect this has on power is modest unless the extent of misclassification bias is substantial; for example, if 5% of controls would meet the definition of cases at the same age, the loss of power is approximately the same as that due to a reduction of the sample size by 10%6. Even for the higher prevalence conditions examined by the WTCCC (such as HT, CAD and T2D), the precise ascertainment schemes used here (which enriched for more extreme phenotypes and/or strong family history) will have limited the proportions of controls meeting case criteria to low levels (for example, to <5%). Although a study design which used ‘hypercontrols’ (that is, selection of control individuals from the lower extremity of
