On measurements and their quality: Paper 1: Reliability - History, issues and procedures

This is the first in a short series of papers on measurement theory and practice with particular relevance to intervention research in nursing, midwifery, and healthcare. In this article I discuss reliability, its origins in classic measurement theory, important issues to consider when operationally defining reliability for a particular study, correlational procedures for assessing the reliability of data once collected, including test-retest reliability, split-half reliability, and Cronbach's coefficient α. Some important insights into reliability provided by attenuation theory are also offered.