Abstract
Psychometric properties of the Insomnia Severity Index (ISI) were analyzed in U.S. college samples. ISI items and total score with sleep and psychosocial questionnaires were examined in Experiment I. ISI diagnostic accuracy in a clinical sample with and without insomnia was assessed in Experiment II. ISI test-retest validity, confirmatory factor analysis (CFA), and item response theory via graded response model (GRM) were assessed in Experiment III. Results indicated analogous ISI and sleep diary items showed moderate correlations (r(1) = .40; r(2) = .45). The ISI total had weak to strong correlations with other indicators of sleep-related disturbance (rs = .25-.62). The ISI had weak to moderate correlations with psychosocial measures commonly associated with insomnia (rs = .10-.57). The diagnostic accuracy of the ISI was very high (area under the curve [AUC] = .999). Sensitivity and specificity were maximized at a cutoff score ≥ 8. The ISI demonstrated good test-retest reliability (ICC = .87). CFA revealed a three-factor model for two study samples and GRM indicated better ability of the ISI to assess moderate (Sample III) and moderate to high (Sample I) levels of insomnia severity. The ISI demonstrated good psychometric properties and appears generally valid for screening insomnia disorder and assessing insomnia severity in college students. Overlap with psychological symptoms suggests caution while interpreting these constructs independently.