Hotz, V. Joseph, Guido W. Imbens, and Jacob A. Klerman (2006). Evaluating the differential effects of alternative welfare-to-work training components: A re-analysis of the California GAIN program, Journal of Labor Economics 24(3): 521-566. Available at https://scholar.harvard.edu/files/imbens/files/evaluating_the_differential_effects_of_alternative_welfare-to-work_training_components_a_re-analysis_of_the_california_gain_program.pdf.