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Questionnaire discrimination: (re)-introducing coefficient d

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posted on 2023-06-07, 14:53 authored by Matthew Hankins
Background: Questionnaires are used routinely in clinical research to measure health status and quality of life. Questionnaire measurements are traditionally formally assessed by indices of reliability (the degree of measurement error) and validity (the extent to which the questionnaire measures what it is supposed to measure). Neither of these indices assesses the degree to which the questionnaire is able to discriminate between individuals, an important aspect of measurement. This paper introduces and extends an existing index of a questionnaire's ability to distinguish between individuals, that is, the questionnaire's discrimination. Methods: Ferguson (1949) [1] derived an index of test discrimination, coefficient d, for psychometric tests with dichotomous (correct/incorrect) items. In this paper a general form of the formula, d G , is derived for the more general class of questionnaires allowing for several response choices. The calculation and characteristics of d G are then demonstrated using questionnaire data (GHQ-12) from 2003–2004 British Household Panel Survey (N = 14761). Coefficients for reliability (a) and discrimination (d G ) are computed for two commonly-used GHQ-12 coding methods: dichotomous coding and four-point Likert-type coding. Results: Both scoring methods were reliable (a > 0.88). However, d G was substantially lower (0.73) for the dichotomous coding of the GHQ-12 than for the Likert-type method (d G = 0.96), indicating that the dichotomous coding, although reliable, failed to discriminate between individuals. Conclusion: Coefficient d G was shown to have decisive utility in distinguishing between the cross-sectional discrimination of two equally reliable scoring methods. Ferguson's d has been neglected in discussions of questionnaire design and performance, perhaps because it has not been implemented in software and was restricted to questionnaires with dichotomous items, which are rare in health care research. It is suggested that the more general formula introduced here is reported as d G , to avoid the implication that items are dichotomously coded.

History

Publication status

  • Published

File Version

  • Published version

Journal

BMC Medical Research Methodology

ISSN

1471-2288

Publisher

BioMed Central

Issue

1

Volume

7

Page range

19

Department affiliated with

  • Primary Care and Public Health Publications

Full text available

  • Yes

Peer reviewed?

  • Yes

Legacy Posted Date

2008-10-24

First Open Access (FOA) Date

2018-04-12

First Compliant Deposit (FCD) Date

2019-07-02

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