Lost in translation? Not anymore: a Mokken scale analysis of the revised Icelandic translation of the PHQ-9

The Patient Health Questionnaire-9 (PHQ-9) is among the most widely used self-report instruments for assessing depressive symptoms. Although previously translated into Icelandic and widely adopted in clinical and research contexts, recent studies identified notable translation issues that may have affected its psychometric quality. This study examined a revised Icelandic version of the PHQ-9, designed to improve clarity and fidelity to the original English version. A general population sample (n = 139) completed the revised version in an online survey. Mokken scale analysis was conducted to evaluate scalability, monotonicity, local independence, and invariant item ordering. Results indicated that the revised version demonstrated stronger psychometric properties compared to the original translation. The overall scalability coefficient suggested a strong scale (H = .548) according to Mokken’s criteria; however, the confidence interval (95% CI [.466, .630]) crossed .50, indicating that strong scalability cannot be guaranteed at the population level. All items surpassed the conventional .30 benchmark, with most achieving Hi ≥ .50. No significant violations of monotonicity were observed, and only minor indications of negative local dependence emerged. While partial evidence for invariant item ordering was found, it remained limited. These findings suggest that the revised Icelandic PHQ-9 is a more internally consistent and interpretable assessment of depression severity, broadly comparable to findings reported in other language versions. The scale appears suitable for use in research settings and for screening purposes in Iceland, although further validation in clinical populations is needed.