Abstract
A response to Norman GR 'Discrimination and reliability: equal partners?' and Wyrwich KW 'Understanding the role of discriminative instruments in HRQoL research: can Ferguson's Delta help?'
Highlights
The examples in the article take validity and reliability as givens; it is assumed that anyone interested in the discrimination of an instrument has already established that the instrument is reliable and valid, by whatever means they find acceptable
What I have argued, is that validity and reliability alone fail to establish that a discriminative instrument achieves its purpose of discriminating between individuals
Norman suggests that the discrimination of an instrument is already indexed by the reliability coefficient, citing his own textbook [3]: 'the reliability coefficient reflects the extent to which a measurement instrument can differentiate among individuals, since the magnitude of the coefficient is directly related to the variability between subjects", and later,' reliability is a measure of the extent to which people differ, expressed as a number between 0 and 1'
Summary
The examples in the article take validity and reliability as givens; it is assumed that anyone interested in the discrimination of an instrument has already established that the instrument is reliable and valid, by whatever means they find acceptable. What I have argued, is that validity and reliability alone fail to establish that a discriminative instrument achieves its purpose of discriminating between individuals. They are not complete examples of the development of a HRQoL instrument (Wyrwich's chief complaint), nor do they suggest that Delta should replace the reliability coefficient (Norman's main objection).
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