Test–retest reliability and validity of the Pittsburgh Sleep Quality Index in primary insomnia
Introduction
Since its introduction in 1989, the Pittsburgh Sleep Quality Index (PSQI) [1] has gained widespread acceptance as a useful tool to measure sleep quality in different patient groups (for overview of sleep questionnaires, see [2], [3]). Its good reliability and validity could be shown for patients with psychiatric and sleep disorders [1], [4], for patients with different somatic diseases [5], for nursing home residents [6], and for healthy elderly subjects [7], whereas data on its validity and reliability for primary insomnia are scarce. Primary insomnia is a sleep disorder that is caused neither by an organic nor by any other psychiatric disorder. There is a chronic difficulty in initiating or maintaining sleep, which causes clinically significant distress or impairment in social, occupational, or other important areas of functioning. Doi et al. [4] evaluated the reliability of a Japanese version of the PSQI in a sample of psychiatric patients, which included a small subsample of 14 patients with primary insomnia. They found a high reliability with an overall Cronbach's alpha of .77 (n=14). There are no data published yet on test–retest reliability and validity of the PSQI with sleep logs for patients with primary insomnia. The study presented here serves this goal.
Section snippets
PSQI
The PSQI is a self-rating questionnaire resulting in a global score between 0 and 21, which consists of seven subscores (see Table 1). The questionnaire is easy to handle and can be completed within 5 min. The whole questionnaire is published by Buysse et al. [1]. The German version has been translated from English into German and then retranslated for comparison with the original version. In the German version, the estimate period was reduced from 4 to 2 weeks [8], because the German PSQI
Results
The mean PSQI global score was 12.5±3.8 for insomnia patients and 3.3±1.8 for controls (P=.000). All PSQI subscores were also significantly different between both groups (P=.000), with higher scores for the insomnia patients. The recommended PSQI global score >5 as an indicator of relevant sleep disturbances [1] resulted in a sensitivity for primary insomnia of 98.7. The specificity was 84.4, with 7 out of 45 healthy subjects having a PSQI global score of 6 and none having a global score beyond
Discussion
For patients with primary insomnia, the overall test–retest reliability for the PSQI global score is high for a short-term interval of 2 days, as well as for a longer interval of several weeks. The fact that some subscore test–retest coefficients diminish when the time interval between the tests is longer underlines the variability of sleep across time.
The correlation of PSQI data with sleep logs demonstrates the good validity of the PSQI. The retrospective evaluation of several weeks' sleep
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