Conducting Measurement Invariance Tests with Ordinal Data: A Guide for Social Work Researchers

Objective: The validity of measures across groups is a major concern for social work researchers and practitioners. Many social workers use scales, or sets of questionnaire items, with ordinal response options. However, a review of social work literature indicates the appropriate treatment of ordinal data in measurement invariance tests is rare; only 3 of 57 articles published in 26 social work journals over the past 12 years used proper testing procedures. This article synthesizes information from the literature and provides recommendations for appropriate measurement invariance procedures with ordinal data. Method: We use data from the Cebu Longitudinal Health and Nutrition Survey to demonstrate applications of invariance testing with ordinal data. Using a robust weighted least squares estimator and polychoric correlation matrix, we examine invariance of a 10-item Perceived Stress Scale (PSS) across 2 young adult groups defined by health status. We describe 2 competing approaches: a 4-step approach, in which factor loadings and thresholds are tested and constrained separately; and a 3-step approach, in which loadings and thresholds are tested and constrained in tandem. Results: Both approaches lead to the same conclusion that the 2 dimensions of the PSS are noninvariant across health status. In the absence of invariance, mean scores on the PSS factors cannot be validly compared across groups, nor should latent variables be used in the hypothesis testing across the 2 groups. Readers are directed to online resources. Conclusions: Careful examination of social work scales is likely to reveal fit or noninvariance problems across some groups. Use of appropriate methods for invariance testing will reduce misuse of measures in practice and improve the rigor and quality of social work publications.