Longitudinal Measurement Invariance of the Brief Symptom Inventory (BSI)-18 in Psychotherapy Patients

The Brief Symptom Inventory (BSI)-18 is a widely-used tool to assess changes in general distress in patients despite an ongoing debate about its factorial structure and lack of evidence for longitudinal measurement invariance (LMI). We investigated BSI-18 scores from 1,081 patients from an outpatient clinic collected after the 2nd, 6th, 10th, 18th, and 26th therapy session. Confirmatory factor analysis (CFA) was used to compare models comprising one, three, and four latent dimensions that were proposed in the literature. LMI was investigated using a series of model comparisons, based on chi-square tests, effect sizes, and changes in comparative fit index (CFI). Psychological distress diminished over the course of therapy. A four-factor structure (depression, somatic symptoms, generalized anxiety, and panic) showed the best fit to the data at all measurement occasions. The series of model comparisons showed that constraining parameters to be equal across time resulted in very small decreases in model fit that did not exceed the cutoff for the assumption of measurement in variance. Our results show that the BSI-18 is best conceptualized as a four-dimensional tool that exhibits strict longitudinal measurement invariance. Clinicians and applied researchers do not have to be concerned about the interpretation of mean differences over time