The degree to which the set of items of a data collection instrument is known to represent the range or universe of meanings or dimensions of a construct of interest. For example, how well do the domains of a health-related quality of life index for rheumatic arthritis represent the aspect of quality of life or daily functioning that are important to patients. focuses on how well a measurement instrument (such as a test, questionnaire, or assessment tool) covers the full range of content or domain that it is intended to measure. In other words, it evaluates whether the items or questions included in the instrument are representative of the entire construct being measured.