Heckman, James J., "Sample Selection Bias as a Specification Error", Econometrica, Vol. 47, No. 1, (January 1979), pp. 153-161.
Abstract: This paper discusses the bias that results from using nonrandomly selected samples to estimate behavioral relationships as an ordinary specification error or "omitted variables" bias. A simple consistent two stage estimator is considered that enables analysts to utilize simple regression methods to estimate behavioral functions by least squares methods. The asymptotic distribution of the estimator is derived.