Background: The psychometric properties of the 3-item Minimal Insomnia Symptom Scale (MISS) have been evaluated using classical test theory. Different cut-offs for identifying insomnia have been suggested in two age groups (≥6 and ≥7 among adult and elderly people, respectively). The aim of this 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 by the Rasch measurement model.

Results: Data generally met Rasch model requirements and the scale could separate between two distinct groups of people, but there was differential item functioning by age. 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 support for the measurement properties of the MISS. However, 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 elderly