Background: The psychometric properties of the three-item Minimal Insomnia Symptom Scale (MISS) were evaluated using the classical test theory. Different cut-offs for identifying insomnia were suggested in two age groups (>6 and >7 among adult and elderly people, respectively). The aim of the present study was to test the measurement properties of the MISS using the Rasch measurement model, with special emphasis on differential item functioning by gender and age.Methods: Cross-sectional MISS data from adult (age 20-64 years, n = 1075) and elderly (age 65+, n = 548) populations were analysed using the Rasch measurement model.Results: Data generally met Rasch model requirements and the scale could separate between two distinct groups of people. Differential item functioning was found by age but not gender. The difference between the adult and elderly samples was lower for the originally recommended >6 points cut-off (0.09 logits) than for the >7 points cut-off (0.23 logits), but greater at the lower and higher ends of the scale. Conclusions: This study provides general support for the measurement properties of the MISS. Caution should be exercised in comparing raw MISS scores between age groups, but applying a >6 cut-off appears to allow for valid comparisons between adults and the elderly regarding the presence of insomnia. Nevertheless, additional studies are needed to determine the clinically optimal cut-score for identification of insomnia.
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