Glasgow Anxiety Scale for people with an Intellectual Disability (GAS-ID): development and psychometric properties of a new measure for use with people with mild intellectual disability

Background Self‐rating scales are widely used in general adult practice; however, there is no reliable and valid method for assessing state anxiety in people with intellectual disability (ID). The present study describes the development and psychometric evaluation of a new scale, the Glasgow Anxiety Scale for People with an Intellectual Disability (GAS‐ID). Methods First, an item pool was generated from focus groups, a review of the literature and clinician feedback. Secondly, a draft scale was administered to 19 anxious and 16 non‐anxious people with ID for further validation and appraisal of reliability. Thirdly, the scale was completed by 19 anxious, non‐ID people for cross‐validation with the Beck Anxiety Inventory (BAI). Finally, physiological concomitants were validated by pulse‐oximetry. Results The 27‐item GAS‐ID discriminated anxious from non‐anxious participants, had good test–retest reliability (r = 0.95) and internal consistency (α = 0.96), and was reasonably correlated with the BAI (ρ = 0.75). The correlation between the physiological subscale of the GAS‐ID and changes in pulse rate was moderately significant (ρ = 0.52). Conclusions This preliminary study suggests that the GAS‐ID offers a psychometrically robust and practical (5–10 min) approach to the appraisal of anxiety in this population.