Item ordering of personal disturbance scale (DSSI/sAD) in a longitudinal study; using Mokken scale analysis

•A large community-based cohort of women were followed for 21 years.•Women’s mental health were assessed 6 times using the personal disturbance scale.•Mokken scale analysis was applied to 14 items of the scale at each time point.•A common hierarchical scale with 10 items was identified.•The scale can be used to measure changes in women’s mental health over time.

In order to establish a common scale with a stable structure over time to use in repeated measures studies, this study uses information from a large community-based cohort of women. Mental health was assessed at six time points during 21 years, using the Delusions-Symptoms-States Inventory/States of Anxiety and Depression (DSSI/sAD) for 2698 women with mean age of 25.2 years at baseline. Mokken scale analysis is applied to the 14 items of DSSI/sAD at each time point, using a hierarchical clustering Algorithm and a Genetic Algorithm. Both algorithms resulted in one Mokken scale at each time point. After excluding items that did not meet the assumptions of Mokken scaling for polytomous data, a single common scale with 10 common items was extracted from scales, which was highly reliable at each time point. This measure with reasonable hierarchy of severity of items (symptoms) has a stable structure over time and can be used to track the changes of women’s mental health over time and analysed via repeated measures analysis.