scores more symmetrically distributed around an intermediate mode. Table 4 shows the specific item parameters simulated for the second test. In addition, the raw trait values and test scores were dichotomized to generate outcome (“disease” phenotypes) at thresholds giving the closest to 20 % prevalence in the population. The raw sum scores for the first test were also subjected to a square root transformation to minimize the effects of heteroscedasticity on the subsequent regression analysis of the raw symptom counts (c.f. Bartlett 1947) and spurious non-additive genetic effects (c.f. Eaves and Eysenck 1977). It will be seen that simple transformation does not always have the desired result.
