Taken overall, the results of testing candidate gene models for epistasis and G × E may be seriously misleading even under the simplest additive genetic model (Model 1) when investigators are forced to analyze test scores based on items with restricted range of difficulty. Dichotomizing scores and trait values avoids much of the potential bias but at the cost of dramatically reduced power in exploratory analysis. The problem is only partly resolved by a square root transformation of test scores but difficulties can be minimized if it is possible to construct a test in which the item parameters span the range of hypothesized trait values.
