This thesis focussed on the psychometric properties of measures of mental wellbeing, for individuals with mild to moderate intellectual disabilities. The aim of the systematic review (Part 1) was to provide an update to previous evaluations of measures of common mental health problems and mental wellbeing, for adults with mild to moderate intellectual disabilities. The psychometric properties of nine measures were assessed using a quality assessment tool and a paucity of psychometrically robust measures of mental wellbeing for this population was observed.

The aim of the empirical paper (Part 2) was to assess the psychometric properties of a newly adapted version of the 14-item Warwick-Edinburgh Mental Wellbeing Scale (WEMWBS; Tennant et al., 2007) and the Short 7-item WEMWBS (SWEMWBS), for individuals with mild to moderate intellectual disabilities (WEMWBS-ID/SWEMWBS-ID). This adapted version was developed by the primary research supervisor, experts by experience and colleagues in the intellectual disability field. The results suggest that the WEMWBS-ID and SWEMWBS-ID have promising psychometric properties. This empirical study was completed jointly with another trainee clinical psychologist, who examined predictors of self-esteem in individuals with intellectual disabilities.

The critical appraisal (Part 3) describes my reflections on the process of completing the systematic review and empirical study. A consideration of how my previous experiences influenced why I chose this project, the specific challenges faced whilst completing both papers and my reflections on quantitative measurement in the field of positive mental health are discussed.