User's Guide to Ratio Variables

Some sociologists intuit a necessary relation between ratio variables with common components, and they warn that observed relations between such variables are likely to be "artifactual." In rebuttal, other sociologists argue that the use of ratio variables is justified when the ratios themselves, rather than their components, are of "theoretical interest." Both arguments are based on mistaken ideas about estimating causal effects in nonexperimental research. To realize unbiased estimates, social scientists often must controlfor a dominant confounding variable Z (e.g., population size). One control procedure is to divide the other variables by Z, i.e., to use ratio variables. Contrary to the "artifactuality" argument, this control procedure does not introduce bias; but contrary to the "theoretical interest" argument, the control procedure need not be restricted to instances where the ratios themselves are of theoretical interest. Conventional wisdom notwithstanding, theoretical interest is not a reliable guide for determining whether or not to use ratio variables.