Content validity of measures of theoretical constructs in health psychology: discriminant content validity is needed

Many of the theoretical constructs and outcomes of interest to health psychology cannot be objectively assessed. For example, phenomena such as beliefs, pain, health, quality of life, stress, intention, illness representations are all of interest but none are available for direct measurement. Rather, the measurement of such theoretical constructs is an inferential process requiring the development of instruments that assess the target construct indirectly, typically using questionnaire‐based measures. Establishing and reporting the psychometric properties of such measures is challenging but fundamental to their utility in testing theory, designing and evaluating interventions and making clinical and policy decisions.