THE POWER OF THE CLASSICAL TWIN STUDY TO RESOLVE VARIATION IN THRESHOLD TRAITS

We explore the power of the twin study to resolve sources of familial resemblance when the data are measured at the binary or ordinal level. Four components of variance were examined: additive genetic, nonaddit ive genetic, and common and specific environment. Curves are Presented to compare the power of the continuous case with those of threshold m odels corresponding to different prevalences in the population: 1,5, 1 0, 25, and 50%. Approximately three times the sample size is needed fo r equivalent power to the continuous case when the threshold is at the optimal 50%, and this ratio increases to about 10 times when 10% are above threshold. Some power may be recovered by subdividing those abov e threshold to form three or more ordered classes, but power is determ ined largely by the lowest threshold. Non-random ascertainment of twin s;(i) through affected twins and examining their cotwins or (ii) throu gh ascertainment of all pairs in which at least one twin is affected i ncreases power. In most cases, strategy i is more efficient than strat egy ii. Though powerful for the rarer disorders, these methods suffer the disadvantage that they rely on prior knowledge of the population p revalence. Furthermore, sampling from hospital cases may introduce bia ses, reducing their value. A useful approach may be to assess the popu lation with a screening instrument; the power calculations indicate th at sampling all concordant and half of the discordant pairs would be e fficient, as along as the cost of screening is not too high.