Definitions of measurement properties
Consensus-based definitions of all included measurement properties are presented in Table 1. A measurement property is a quality aspect of an outcome measure. Each measurement property requires its own type of study to assess it.
1 Content validity is considered to be the most important measurement property because first of all it should be clear that all content (e.g. items, tasks, observations or parameters) of an outcome measurement instrument is relevant, comprehensive, and comprehensible with respect to the construct of interest and target population.
2 Next, we consider the internal structure of an instrument to be important, including the measurement properties structural validity, internal consistency, and cross-cultural validity\measurement invariance. Internal structure refers to how the different aspects (e.g. items, tasks, observations or parameters) in an outcome measurement instrument are related, which is important to know for deciding how these aspects might be combined into a scale or subscale. Evaluating the internal structure of the instrument is relevant for outcome measures that are based on a reflective model. In a reflective model the construct manifests itself in the aspects, i.e. the items or tasks are a reflection of the construct to be measured . Its counterparts is a formative model, in which the aspects within an outcome measure are not supposed to be correlated. Analyses on the internal structure of outcome measures based on formative models can be ignored.
3 Finally, the remaining measurement properties are considered important, i.e. reliability, measurement error, criterion validity, hypotheses testing for construct validity and responsiveness.