Abstract
BACKGROUND: The Four-Dimensional Symptom Questionnaire (4DSQ) (Huisarts Wetenschap 39: 538-47, 1996) is a self-report questionnaire developed in the Netherlands to distinguish non-specific general distress from depression, anxiety, and somatization. This questionnaire is often used in different populations and settings and there is a paper-and-pencil and computerized version. METHODS: We used item response theory to investigate whether the 4DSQ measures the same construct (structural equivalence) in the same way (scalar equivalence) in two samples comprised of primary mental health care attendees: (i) clients who visited their General Practitioner responded to the 4DSQ paper-and-pencil version, and (ii) eHealth clients responded to the 4DSQ computerized version. Specifically, we investigated whether the distress items functioned differently in eHealth clients compared to General Practitioners' clients and whether these differences lead to substantial differences at scale level. RESULTS: Results showed that in general structural equivalence holds for the distress scale. This means that the distress scale measures the same construct in both General Practitioners' clients and eHealth clients. Furthermore, although eHealth clients have higher observed distress scores than General Practitioners' clients, application of a multiple group generalized partial credit response model suggests that scalar equivalence holds. CONCLUSIONS: The same cutoff scores can be used for classifying respondents as having low, moderate and high levels of distress in both settings.