Abstract
BACKGROUND: The nine-item Physical Health Questionnaire (PHQ-9), the seven-item Generalised Anxiety Disorder scale (GAD-7) and the Work and Social Adjustment Scale (WSAS) are, respectively, self-report measures of depression, generalised anxiety, and the impact of mental health on the person's personal functioning that are widely used in mainstream mental health services in England. The psychometric properties of these scales when used with people with intellectual disabilities have not been established. METHOD: Item level data for the PHQ-9 (n = 128), GAD-7 (n = 124) and WSAS (n = 133) for people with intellectual disabilities in an English NHS Talking Therapies for anxiety and depression (NHSTT) service in the north of England were analysed using internal reliability statistics and confirmatory factor analysis. RESULTS: In this study, the full PHQ-9, GAD-7 and WSAS have Cronbach's α of 0.81, 0.84 and 0.81, respectively, and have acceptable ranges of corrected item-total correlations. The two-factor structures for the PHQ-9 and the GAD-7 were a better fit than single-factor structures, although the single-factor fit and the correlation between the two factors within each scale suggest that their use as a single scale is justified. The single-factor structure for the WSAS was a good fit. CONCLUSIONS: In this study, the widely used PHQ-9, GAD-7 and WSAS demonstrate internal consistency values and factor analysis structure similar to those for individuals without intellectual disabilities. The data support the use of these measures for people with intellectual disabilities attending routine primary care mental health services.