Does item homogeneity indicate internal consistency or item redundancy in psychometric scales?

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Abstract

The term ‘internal consistency’ has been used extensively in classical psychometrics to refer to the reliability of a scale based on the degree of within-scale item intercorrelation, as measured by say the split-half method, or more adequately by Cronbach's (1951) (Psychometrika, 16, 297–334) alpha, as well as the KR20 and KR21 coefficients. This term is a misnomer, as a high estimate of internal item consistency/item homogeneity may also suggest a high level of item redundancy, wherein essentially the same item is rephrased in several different ways.

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      For some of these measures, Cronbach’s alpha fell below conventional standards for adequate internal consistency (α < 0.70), but many have argued that McDonald’s omega is a better measure of internal consistency (McDonald, 1999; Revelle & Zinbarg, 2008; Sijtsma, 2009; Zinbarg et al., 2005). Perhaps more importantly, modest internal consistencies are not only expected but preferred when measuring a broad content space with a relatively brief questionnaire (Boyle, 1991; Kline, 1979), as high internal consistency may indicate that the brief measure is redundant or too highly focused and, thus, fails to adequately cover the full breadth of the construct (Kline, 1986, pp. 118). Prior to conducting analyses, data were inspected for meeting the statistical assumption of normality.

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