Problems with measuring socioeconomic status (SES)-frequently included in clinical and public health studies as a control variable and less frequently as the variable(s) of main interest-could affect research findings and conclusions, with implications for practice and policy. We critically examine standard SES measurement approaches, illustrating problems with examples from new analyses and the literature. For example, marked racial/ethnic differences in income at a given educational level and in wealth at a given income level raise questions about the socioeconomic comparability of individuals who are similar on education or income alone. Evidence also shows that conclusions about nonsocioeconomic causes of racial/ethnic differences in health may depend on the measure-eg, income, wealth, education, occupation, neighborhood socioeconomic characteristics, or past socioeconomic experiences-used to "control for SES," suggesting that findings from studies that have measured limited aspects of SES should be reassessed. We recommend an outcome- and social group-specific approach to SES measurement that involves (1) considering plausible explanatory pathways and mechanisms, (2) measuring as much relevant socioeconomic information as possible, (3) specifying the particular socioeconomic factors measured (rather than SES overall), and (4) systematically considering how potentially important unmeasured socioeconomic factors may affect conclusions. Better SES measures are needed in data sources, but improvements could be made by using existing information more thoughtfully and acknowledging its limitations.

Socioeconomic factors play an important but complex role in PTB disparities. The absence of Black-White disparities in PTB within certain socioeconomic subgroups, alongside substantial disparities within others, suggests that social factors moderate the disparity. Further research should explore social factors suggested by the literature-including life course socioeconomic experiences and racism-related stress, and the biological pathways through which they operate-as potential contributors to PTB among Black and White women with different levels of social advantage.

Conclusions about the role of race/ethnicity could vary with how SES is measured. Education is not an acceptable proxy for income in studies of ethnically diverse populations of childbearing women. SES measures generally should be outcome- and population-specific, and chosen on explicit conceptual grounds; researchers should test multiple theoretically appropriate measures and consider how conclusions might vary with how SES is measured. Researchers should recognize the difficulty of measuring SES and interpret findings accordingly.