The validity of a diagnosis is more difficult to ascertain than its reliability. If a classification is to be valid ('based on truth or correct reasoning'), the categories


it contains should describe disorders, which really are separate from one another. Various types of validity have been distinguished, such as face validity, which is to do with whether the diagnosis 'looks right', that is, whether it appears, on inspection, to deal with the matters it says it is going to deal with.

Ways of testing the validity of diagnostic groupings include the following:

• examining the consistency of symptom patterns. Statistical techniques such as 'cluster analysis' and 'discriminant function analysis' facilitate this process

• demonstration of consistent genetic and biological correlates

• demonstration of a consistent natural history and long-term outcome

• demonstration of a consistent treatment response.

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