VALIDATION DATA

Cronbach’s α was 0.74 in the original validation study. A subsequent population-based study of 959 individuals showed high internal consistency, with a Cronbach’s α of 0.90 and 0.91 for community and clinical samples, respectively.

Item-total correlations ranged from 0.50 to 0.85. Items about satisfaction and worry showed higher item-total correlations, whereas items about difficulty initiating sleep or early-morning awakening showed lower correlations

Pearson’s correlation was r = 0.83 after 1 month, r = 0.77 after 2 months, and r = 0.73 after 3 months in a cancer cohort

The items in the ISI correspond to many of the criteria for insomnia in the DSM-IV, as well as the updated Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), including difficulty initiating or maintaining sleep, dissatisfaction in sleep, early-morning awakening, interference with daytime functioning, and distress related to the sleep disturbance. In the original validation study, a principal component analysis explored the ISI content validity. This analysis yielded three components that explained 72% of the total variance. Component I included items related to daytime interference, noticeability by others, and level of distress. Component II related to severity of sleep onset, maintenance, and early-morning awakening difficulties. Component III included satisfaction with sleep and severity of initial insomnia