The ability of an instrument (method, questionnaire) to measure what it has to measure (or what we think or expect to measure). This is what is called “truth” in the OMERACT filter. The validity of an instrument is not obvious. For example: RA patients do have high scores on some depression questionnaires. Not because they are depressed, but as the results of questions like: “I always have a slow start in the morning”, “I often feel tired” and “I can’t do the same as before”. For this reason one has to conclude that such a questionnaire is not valid for RA patients.

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