Establishing a Caseness Cutoff and Reliable Change Index for the Outcome Measurement Scale

Background: Routine Outcome Monitoring (ROM) is an evidence-based approach that enhances psychotherapy outcomes by systematically tracking client progress. However, its implementation in real-world practice—particularly in non-Western settings—remains limited, partly due to a lack of brief and interpretable outcome measures. The Outcome Measurement Scale (OMS) was developed to address this gap by assessing both psychological distress and multidimensional well-being in South Korea. This study aimed to establish two key interpretive anchors for the OMS: a caseness cutoff score and a Reliable Change Index (RCI) to support its clinical and public health applications. Methods: A sample of 1,400 adults from South Korea completed both the OMS and the WHO-5 Well-Being Index, a validated measure of subjective well-being. Equipercentile linking and Receiver Operating Characteristic (ROC) analysis were employed to identify the OMS score corresponding to the WHO-5 caseness threshold (< 13). The RCI was calculated using the Jacobson–Truax method based on estimated measurement error and internal consistency reliability. Results: Equipercentile linking indicated that a WHO-5 score of 13 corresponded to an OMS score of 33, while ROC analysis identified 31 as the optimal cutoff (AUC = 0.90; sensitivity = 0.88; specificity = 0.27). The OMS and WHO-5 demonstrated strong correlation ( r = 0.80). The RCI was calculated as 8.11 points for the full sample and 6.77 points for the clinical subsample, suggesting that a change of approximately 7–8 points reflects statistically reliable improvement or deterioration at the 95% confidence level. Conclusion: An OMS score of approximately 31 serves as a practical caseness threshold, while a 7–8 point change represents reliable individual change. These interpretive benchmarks enable clinicians and public health professionals to screen for low well-being, track meaningful change, and evaluate service outcomes using the OMS. The findings highlight the OMS as a brief and useful tool for ROM implementation in both clinical and community mental health settings.