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A note on using alpha and stratified alpha to estimate the reliability of a test composed of item parcels

Biomedical Sciences Research Institute Computer Science Research Institute Environmental Sciences Research Institute Nanotechnology & Advanced Materials Research Institute

Rae, Gordon (2008) A note on using alpha and stratified alpha to estimate the reliability of a test composed of item parcels. BRITISH JOURNAL OF MATHEMATICAL & STATISTICAL PSYCHOLOGY, 61 (Part 2). pp. 515-525. [Journal article]

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DOI: 10.1348/000711005X72485

Abstract

Several authors have suggested that prior to conducting a confirmatory factor analysis it may be useful to group items into a smaller number of item `parcels' or `testlets'. The present paper mathematically shows that coefficient alpha based on these parcel scores will only exceed alpha based on the entire set of items if W, the ratio of the average covariance of items between parcels to the average covariance of items within parcels, is greater than unity. If W is less than unity, however, and errors of measurement are uncorrelated, then stratified alpha will be a better lower bound to the reliability of a measure than the other two coefficients. Stratified alpha are also equal to the true reliability of a test when items within parcels are essentially tau-equivalent if one assumes that errors of measurement are not correlated.

Item Type:Journal article
Faculties and Schools:Faculty of Life and Health Sciences
Faculty of Life and Health Sciences > School of Psychology
Research Institutes and Groups:Psychology Research Institute
Psychology Research Institute > Health and Wellbeing
ID Code:1735
Deposited By:Mrs Fiona Harkin
Deposited On:23 Dec 2009 10:16
Last Modified:24 Feb 2014 10:44

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