Developing a validity testing plan for a patient-reported outcome measure in a new context using the argument-based approach to validity.

Health-related measures, such as Patient-Reported Outcome Measures (PROMs), are widely used in clinical practice, research, service evaluation, and decision-making. Validity is about the extent to which a score interpretation is supported by evidence for a specific use in a specific context. The argument-based approach to validity provides a systematic way of planning and undertaking validity testing and is gaining attention in health-related measurement, yet few practical examples of its application exist. This commentary firstly describes the phases and steps of the argument-based approach to validity. Secondly, it brings the theory into practice by presenting a worked example that illustrates how to apply the argument-based approach to validity to develop a validity testing plan for the application of a widely used health literacy PROM in a new context. Importantly, the validity testing plan outlines the underpinning assumptions that need investigation to determine the extent to which we can have confidence in the interpretation and use of Health Literacy Questionnaire (HLQ) scores in the New South Wales prison context. The commentary offers a practical exemplar for identifying assumptions, evidence needs and methodological options to support validity testing and to develop an evidence-based argument to verify score interpretation and use in a specific context. The validity testing planning process described and demonstrated in this paper will support the uptake of this approach and promote scientific rigour in health-related measurement.