Which measure of variability is used for nominal variables such as race and ethnicity?

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From: Considerations for using race and ethnicity as quantitative variables in medical education research

Role of the R & E variable	Purpose of R & E variable	Sample medical education research question using R & E variables
Grouping	To examine similarities or differences between R or E groups and/or subgroups based on a dependent (outcome) variable	Is there a significant difference in medical students’ access to professional mentors by R or E group?
Mediating	To examine whether R or E explains the relationship between an independent (predictor) and dependent (outcome) variable	Is the association between socioeconomic status and students’ perceptions of the medical school learning environment reduced when R or E are considered?
Moderating	To examine whether the strength of the relationship between an independent (predictor) and dependent variable (outcome) varies by R or E groups	Does the relationship between social support and well-being vary by R or E group?