The Insomnia Severity Index: cross-cultural adaptation and psychometric evaluation of a Persian version

The main purpose of this study was to describe the cultural adaptation of a Persian version of the Insomnia Severity Index and to evaluate its psychometric properties by testing internal consistency, construct validity, discriminant validity, and concurrent validity.

Cross-cultural adaptation was carried out according to guidelines and included forward and backward translations, expert committee, and field test. A sample of 1,037 sleep clinic patients filled in a questionnaire package including the Persian version of the Insomnia Severity Index (ISI-P), the Pittsburgh Sleep Quality Index (PSQI), the Epworth Sleepiness Scale (ESS), and the Beck Depression Inventory (BDI). In addition, 624 of these patients were assessed by polysomnography (PSG). A group of 50 members of hospital staff responded to the ISI-P as the control group. Internal consistency with Cronbach's alpha, construct validity with principal component analysis, discriminant validity with assessing the ISI-P's ability to differentiate patients from control group and concurrent validity with exploring the spearman correlation with the PSQI, ESS, BDI, and corresponding parameters in PSG were followed to examine the psychometric properties of the ISI-P.

The ISI-P showed acceptable internal consistency (Cronbach's alpha = 0.78) and a 2-factor solution in factor analysis. The ISI-P discriminated well between patients and control group (15.90 ± 6.12 vs 10.10 ± 5.76, P value <0.001). Items and total score of the ISI-P showed a weak correlation with the corresponding parameters in PSG. The ISI-P total score was significantly correlated with total score of the PSQI (r = 0.74, P value <0.001) and the BDI (r = 0.42, P value <0.001), while it was not significantly correlated with total score of the ESS (r = 0.12, P value = 0.72).

Cross-cultural adaptation was successful, and the ISI-P has good psychometric properties. Using the ISI-P is recommended to evaluate insomnia in research and clinical settings in Persian-speaking people.