Measurement of Outcome
In developing an outcome measure the concepts of validity, reliability, and responsiveness are vital.
Validity is the extent to which a questionnaire measures what is intended. This usually is assessed by comparing a new measure with an established one (this is known as criterion validity). In the absence of a gold standard, construct validity can be measured; this allows comparison of a new tool with objective or clinical findings.
Reliability is the degree to which measurements on the same individual are similar under different conditions. Test-retest comparisons are the most appropriate method for assessing reliability if the instrument is intended as an evaluative tool. Reliability can also be assessed using internal consistency; this checks the extent to which similar questions give consistent replies.10
Responsiveness considers whether the tool is sensitive to assess measurable change. If meaningful comparisons are to be made then a standardized measure of responsiveness is required. The standardized response mean allows such a comparison (this represents the mean change in score over two points in time divided by the standard deviation of the score differences).11
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