Psychometric properties of a sleep questionnaire for use in individuals with intellectual disabilities.

We examined the psychometric properties of one part of the Sleep Questionnaire developed by Simonds and Parraga (SQ-SP; 1982), a questionnaire that is frequently used to explore sleep problems and behaviors related to sleep in individuals with intellectual disability (ID). The SQ-SP was completed for 345 individuals with ID (sleep clinic n = 146; control group n = 103; published studies n = 68; psychiatric clinic n = 28). Internal consistency was good (Cronbach's α = .80) and test-retest reliability for the total SQ-SP score was also good (Spearman's rank correlation = .83, p<.01). Convergent validity was adequate (r = .79, p<.001) and concurrent validity was satisfactory (r = .52, p<.001). Exploratory factor analysis suggested a 5-factor structure (Snoring, Daytime sleepiness, Complaints related to sleep, Sleep apnea and Anxiety related to sleep). Internal consistency of the five factors ranged from modest (Cronbach's α = .57) to good (Cronbach's α = .82). Confirmatory factor analysis corroborated the 5-factor structure. The Composite Sleep Index, the total SQ-SP score and the factor scores on Daytime Sleepiness and Complaints related to sleep were able to differentiate the control group from the sleep clinic group. The SQ-SP appears to be a reliable and valid tool in assessing sleep and different types of sleep disturbance in individuals with ID.