Genotypes, environments and liabilities were simulated for a large number of independent subjects (N = 100,000) under a variety of configurations for the additive and non-additive effects of the loci and covariate. Simulated subjects were scored using simulated responses to dichotomous items (k = 20) of checklists using two types of test analogous to those frequently encountered in behavioral measurement. The first, resembling a typical checklist of relatively infrequent symptoms, comprises items with equal low endorsement frequency (difficulty) and the same discriminating power. The second, more characteristic of tests used to assess abilities, comprises items with a wide range of difficultly and variable discriminating power.
