Endogenous Stratification in Randomized Experiments

44 Pages Posted: 20 Dec 2013

See all articles by Alberto Abadie

Alberto Abadie

Harvard University - Harvard Kennedy School (HKS); National Bureau of Economic Research (NBER)

Matthew M. Chingos

Harvard University

Martin R. West

Harvard Graduate School of Education

Date Written: December 2013

Abstract

Researchers and policy makers are often interested in estimating how treatments or policy interventions affect the outcomes of those most in need of help. This concern has motivated the increasingly common practice of disaggregating experimental data by groups constructed on the basis of an index of baseline characteristics that predicts the values that individual outcomes would take on in the absence of the treatment. This article shows that substantial biases may arise in practice if the index is estimated, as is often the case, by regressing the outcome variable on baseline characteristics for the full sample of experimental controls. We analyze the behavior of leave-one-out and repeated split sample estimators and show they behave well in realistic scenarios, correcting the large bias problem of the full sample estimator. We use data from the National JTPA Study and the Tennessee STAR experiment to demonstrate the performance of alternative estimators and the magnitude of their biases.

Suggested Citation

Abadie, Alberto and Chingos, Matthew M. and West, Martin R., Endogenous Stratification in Randomized Experiments (December 2013). NBER Working Paper No. w19742. Available at SSRN: https://ssrn.com/abstract=2370198

Alberto Abadie (Contact Author)

Harvard University - Harvard Kennedy School (HKS) ( email )

79 John F. Kennedy Street
Cambridge, MA 02138
United States
617-496-4547 (Phone)
617-495-2575 (Fax)

National Bureau of Economic Research (NBER)

1050 Massachusetts Avenue
Cambridge, MA 02138
United States

Matthew M. Chingos

Harvard University ( email )

1875 Cambridge Street
Cambridge, MA 02138
United States
617-872-5413 (Phone)

HOME PAGE: http://www.people.fas.harvard.edu/~chingos/

Martin R. West

Harvard Graduate School of Education ( email )

6 Appian Way
Cambridge, MA 02138
United States

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