e outcome, and the
others are pre-treatment covariates.
</p>


<h3>References</h3>

<p>Lalonde, R. (1986). Evaluating the econometric evaluations of
training programs with experimental data. <em>American Economic Review</em> 76:
604-620.
</p>
<p>Dehejia, R.H. and Wahba, S. (1999).  Causal Effects in Nonexperimental
Studies: Re-Evaluating the Evaluation of Training Programs.  <em>Journal of the
American Statistical Association</em> 94: 1053-1062.
</p>

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