Nonrestorative sleep scale: a reliable and valid short form of the traditional Chinese version

PURPOSE: Previous research has suggested the essential unidimensionality of the 12-item traditional Chinese version of the Nonrestorative Sleep Scale (NRSS). This study aimed to develop a short form of the traditional Chinese version of the NRSS without compromising its reliability and validity. METHODS: Data were collected from 2 cross-sectional studies with identical target groups of adults residing in Hong Kong. An iterative Wald test was used to assess differential item functioning by gender. Based on the generalized partial credit model, we first obtained a shortened version such that further shortening would result in substantial sacrifice of test information and standard error of measurement. Another shortened version was obtained by the optimal test assembly (OTA). The two shortened versions were compared for test information, Cronbach’s alpha, and convergent validity. RESULTS: Data from a total of 404 Chinese adults (60.0% female) who had completed the Chinese NRSS were gathered. All items were invariant by gender. A 6-item version was obtained beyond which the test performance substantially deteriorated, and a 9-item version was obtained by OTA. The 9-item version performed better than the 6-item version in test information and convergent validity. It had discrimination and difficulty indices ranging from 0.44 to 2.23 and − 7.58 to 2.13, respectively, and retained 92% of the test information of the original 12-item version. CONCLUSION: The 9-item Chinese NRSS is a reliable and valid tool to measure nonrestorative sleep for epidemiological studies. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s11136-020-02523-4) contains supplementary material, which is available to authorized users.