A Common Measurement Scale for Self-Report Instruments in Mental Health Care

Abstract. The diversity of measures in clinical psychology hampers a straightforward interpretation of test results, complicates communication with the patient, and constitutes a challenge to the implementation of measurement-based care. In educational research and assessment, it is common practice to convert test scores to a common metric, such as T scores. We recommend applying this also in clinical psychology and propose and test a procedure to arrive at T scores approximating a normal distribution that can be applied to individual test scores. We established formulas to estimate normalized T scores from raw scale scores by regressing IRT-based θ scores on raw scores. With data from a large population and clinical samples, we established crosswalk formulas. Their validity was investigated by comparing calculated T scores with IRT-based T scores. IRT and formulas yielded very similar T scores, supporting the validity of the latter approach. Theoretical and practical advantages and disadvantages of both approaches to convert scores to common metrics and alternative approaches are discussed. Provided that scale characteristics allow for their computation, T scores will help to better understand measurement results, which makes it easier for patients and practitioners to use test results in joint decision-making about the course of treatment.