The psychometric structure of the Brief Symptom Inventory–18 (BSI-18; Derogatis, 2001) was investigated using Mokken scaling and parametric item response theory. Data of 487 outpatients, 266 students, and 207 prisoners were analyzed. Results of the Mokken analysis indicated that the BSI-18 formed a strong Mokken scale for outpatients and prisoners, indicating strong unidimensionality. For students, only the depression and anxiety items formed a medium Mokken scale. Parametric item response theory analyses showed that the best discriminating items came from the depression and anxiety subscales.
Meijer, R. R., de Vries, R. M., & van Bruggen, V. (2011). An evaluation of the brief symptom inventory-18 using item response theory: which items are most strongly related to psychological distress? Psychological assessment, 23(1), 193-202. https://doi.org/10.1037/a0021292