Validity

Measures such as patient-reported outcome measures (PROMs), used within clinical trials or service evaluation, should be reliable, valid and sensitive to detect change. These are key psychometric requirements of such tools. Validity is an overarching term referring to whether an instrument measures what is claims to measure. PROMs are used to measure a construct that is not directly observable, such as quality of life or pain. Validity is, therefore, important because in research we rely on the instrument to measure these constructs, rather than something else. There are different types of validity. The main types are: face validity, content validity, construct validity and criterion validity. Face validity refers to whether a measure is perceived by respondents (e.g. patients within a clinical trial) to measure what it says it measures. Low face validity may result in a lower response rate because the instrument may be perceived to lack credibility, for example. Content validity is similar but refers to how well the items within the measure cover the construct of interest (e.g. do they cover all of the domains proposed to underlie quality of life). Construct validity refers to the extent to which items within the measure perform as expected (e.g. whether people respond similarly to items that are designed to measure the same or related constructs). Finally, criterion validity refers to the extent to which scores derived from the measure correlate with other outcomes, in the direction anticipated.

This definition refers to validity within PROMs. Other types of validity can include statistical validity and model validity. Model vality is often separated into ‘internal validity’ (i.e. whether the internal working of the model performs as described in the methods) and ‘external validity’ (i.e. whether the outcomes of the model are a reasonable representation of the ‘real world’).