Development and Validation of the Outcomes of WeLlbeing and Distress Scale for Adults With an Intellectual Disability...

The Psychological Therapies Outcome Scale-Intellectual Disabilities-II (PTOS-ID-II) is a 29-item self-report measure developed to assess psychological distress and positive well-being in people with an intellectual disability. While initial validation demonstrated promising psychometric properties, further replication in a larger sample was needed, and incorporating insights gained from routine clinical use over the years since its initial implementation.

A quantitative cross-sectional design was utilised to test the psychometric properties of the PTOS-ID-II using data collected from routine clinical practice from adults with intellectual disabilities (n = 879) accessing a community health service. Subsequent analyses included dimensionality reduction (via principal component analysis and confirmatory factor analysis), assessment of internal consistency, concurrent validity and receiver operating characteristic analysis. Analyses were guided by a primer on the development of health outcome measures.

Exploration of the proposed factor structure of the PTOS-ID-II indicated that two of the items were problematic and subsequently removed. The new 27-item measure was rebranded 'the Outcomes for Wellbeing and Distress Scale' (OWLS-ID). Analysis of the measure identified and confirmed a three-component model: (1) Positive Well-being, (2) Emotional and Behavioural Discomfort and (3) Anxiety. Internal consistency was good to acceptable. Items related to Emotional and Behavioural Discomfort and Anxiety were combined to create a measure of psychological distress. Concurrent validity between distress as measured using the OWLS-ID and Brief Symptoms Inventory was strong. A cut of 11.5 had acceptable sensitivity (0.85) and specificity (0.9). Analysis of missing data suggested that item acceptability was high.

The OWLS-ID is one of the most comprehensively tested patient-reported outcome measures to date, which was developed specifically for individuals with an intellectual disability. Its implications for clinical practice and future research are discussed.