Omitted variable bias (OVB) occurs when an important independent variable is excluded from an estimation model, such as a linear regression, and its exclusion causes the estimated effects of the included independent variables to be biased. Bias will occur when the excluded variable is correlated with one or more of the included variables. An example of this occurs when investigating the returns to education. This typically involves regressing the log of wages on the number of years of completed schooling as well as on other demographic characteristics such as an individual’s race and gender. One important variable determining wages, however, is a person’s ability. In many such regressions, a measure of ability is not included in the regression (or the measure included only imperfectly controls ...