A psychometric analysis was conducted to reduce the number of items needed to assess the disability associated with mental disorders using the World Health Organization Disability Assessment Schedule (WHODAS). The WHODAS was to be used in the Substance Abuse and Mental Health Services Administration National Survey on Drug Use and Health (NSDUH), beginning in 2008, as part of a screening algorithm to produce estimates of the prevalence of serious mental illness (SMI) in the US adult population. The goal of the work presented in this paper was to create a parsimonious screening scale from the full 16-item WHODAS that was administered to 24156 respondents (aged 18+) in the 2002 NSDUH. Exploratory factor analysis showed that WHODAS responses were unidimensional. A two-parameter polytomous Item Response Theory model showed that all 16 WHODAS items had good item discrimination (slopes greater than 1.0) for each response option. Analysis of item difficulties and differential item function across socio-demographic categories was then used to select a subset of eight items to create a short version of the WHODAS. The Pearson correlation between scores in the original 16-item and reduced eightitem WHODAS scales was 0.97, documenting that the vast majority of variation in total scale scores was retained in the reduced scale. Copyright © 2010 John Wiley & Sons, Ltd.
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