Uncovering measurement bias: Differential item functioning by income in the Couples Satisfaction Index

Relationship satisfaction is among the most popular constructs in family science. As the study of families and romantic couples continues to include more diverse samples, it is imperative to ensure the measures scholars use do not significantly vary in psychometric quality across groups. The goal of this study was to examine the psychometric utility of the four-item Couples Satisfaction Index (CSI-4; Funk & Rogge, 2007) for individuals across the income spectrum. This study uses data from the National Couples' Health and Time Study, which sampled married and cohabiting adults in the United States from late 2020 to early 2021. Differential item functioning in the CSI-4 was assessed based on income (N = 3,497) using item response theory. For individuals experiencing lower incomes, the CSI-4 demonstrated differential functioning compared to their more affluent peers, with a bias against individuals experiencing lower incomes equating to average scores 1.38 units lower than higher income participants on the CSI-4 despite having the same true levels of relationship satisfaction. Differential item functioning was less pronounced when comparing item performance of lower- and middle-income participants, equating to lower income participants scoring .21 units lower than middle-income individuals despite having the same true levels of relationship satisfaction. While advancing diversity in family science, researchers should employ psychometric methods to understand how measures perform across groups, which will help prevent the misinterpretation of statistical bias as real group differences. (PsycInfo Database Record (c) 2024 APA, all rights reserved).