Reliability and validity of screening scales: effect of reducing scale length.

Self-report measures are often useful as the first phase of a multiphase case identification procedure for estimating rates of untreated disorder, but such measures are susceptible to several sources of unreliability. The reliability of respondents' reports can be greatly improved by employing a well-established screening scale composed of multiple replicate items. A practical question that arises is whether interview time can be saved by using only a portion of the original scale. The cost of shortening established scales in terms of the sensitivity and specificity of the screen is modelled statistically, and the robustness of the model is assessed using mental health data obtained on samples of psychiatric patients and community controls. Both the statistical model and the empirical examples suggest that items from highly reliable measures can be dropped without much loss in sensitivity or specificity. Suggestions are made for selecting subsets of items when shortening a screening scale and these are illustrated with screening scales for depression.