Development and psychometric properties of the Psychological Therapies Outcome Scale – Intellectual Disabilities (PTOS-ID)

Abstract

Background: There are few valid and reliable psychological therapy outcome measures available for use with people with intellectual disabilities (ID). The current study involved the development of a new scale; the Psychological Therapies Outcome Scale – Intellectual Disabilities (PTOS-ID), and the examination of its validity and internal consistency. Method: The PTOS-ID was administered to 175 people who have ID accessing specialist ID services. The construct validity of the scale was investigated through exploratory factor analysis, concurrent validity through comparison with the Brief Symptom Inventory and internal reliability through internal consistency analysis. Results: Three factors emerged from the principal components analysis with high levels of internal consistency: (1) anger and mood (α = 0.82); (2) positive well-being (α = 0.81); and (3) anxiety (α = 0.76). Factors (1) and (2) were combined to measure psychological distress (α = 0.85), which correlated strongly with the Global Severity Index of the Brief Symptom Inventory (r = 0.85). Conclusions: This preliminary study suggests that the PTOS-ID is a psychometrically robust measure of psychological distress and psychological well-being that can be used with people with ID. Further research is required to assess its reliability and ability to detect change.