Massachusetts Institute of Technology Department of Economics Working Paper Series Quantile Regression under Misspecification with an Application to the Libraries Quantile Regression under Misspecification, with an Application Wage Structure

Quantile regression (QR) fits a linear model for conditional quantiles, just as ordinary least squares (OLS) fits a linear model for conditional means. An attractive feature of OLS is that it gives the minimum mean square error linear approximation to the conditional expectation function even when the linear model is misspecified. Empirical research using quantile regression with discrete covariates suggests that QR may have a similar property, but the exact nature of the linear approximation has remained elusive. In this paper, we show that QR can be interpreted as minimizing a weighted mean-squared error loss function for specification error. The weighting function is an average density of the dependent variable near the true conditional quantile. The weighted least squares interpretation of QR is used to derive an omitted variables bias formula and a partial quantile correlation concept, similar to the relationship between partial correlation and OLS. We also derive general asymptotic results for QR processes allowing for misspecification of the conditional quantile function, extending earlier results from a single quantile to the entire process. The approximation properties of QR are illustrated through an analysis of the wage structure and residual inequality in US census data for 1980, 1990, and 2000. The results suggest continued residual inequality growth in the 1990s, primarily in the upper half of the wage distribution and for college graduates. Acknowledgment. We thank David Autor, Gary Chamberlain, George Deltas, Jinyong Hahn, Jerry Hausman, Roger Koenker, and Art-Lewbel for helpful discussions, and seminar participants at BYU, the University of Michigan, Michigan State University, the Harvard-MIT Econometrics Workshop, the University of Toronto, the University of Illinois at UrbanaChampaign, and the 2004 Winter Econometric Society Meetings for comments.