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This is very much an ongoing procedure that requires a number of studies of the performance of a questionnaire in a range of settings and patient groups.Each one of these studies will examine some aspect of the validity of particular constructs or ‘mini-theories.’ It may be, for example, that responses to the questionnaire are compared with clinical tests confirming a diagnosis, or in different age groups if an age relationship is postulated.A Cronbach’s alpha in excess of 0.70 is usually considered to show adequate internal consistency.
Construct validity also includes the concepts of ‘convergent’ and ‘discriminant’ validity.
Criterion validity describes how well the questionnaire correlates with a 'gold standard' measure that already exists.
Such ‘gold standards’ may be clinical or other validated measures.
Where a questionnaire results in a simple score, treatment effects can be assessed by examining pre- and post-treatment differences between the intervention and control group by means of unpaired t-tests or repeated measures analysis of variance.
As additional evidence, patients’ perceptions of change can also be measured and relationships between reported change and difference in quality of life scores can be examined. Outside randomised controlled trials, where there may be baseline differences between treatment groups, analyses of covariance may be more appropriate for assessing responsiveness.
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There has been considerable controversy concerning the most appropriate methods of measuring change in questionnaires.