many psychopathology scales have only items that refer to relatively rare symptoms. As a consequence, many individuals in the general population score 0, which does not necessarily imply that all actually have the trait to the exact same degree. In other words, the scale provides very little information on the trait on the low end. In contrast, the upper end of the scale usually shows more variation, which may imply that the measures are more reliable (more items that discriminate between individuals). Thus, a priori it seems likely that psychopathological scales have more discriminatory power at the upper end of the scale than at the lower end. Of course, this is not a bad thing, since these scales were designed to discriminate between the healthy and the sick. Therefore it seems reasonable to forego the assumption of equal reliability across the scale and take differing reliabilities into account.
