求助两篇英文文献，谢谢大家！

第一篇：

作者：Heckman, J. and R. Robb

题目：Alternative methods for evaluating the impact of interventions

来源：  in: J. Heckman and B. Singer, eds., Longitudinal analysis of labor market data (Cambridge University Press for Econometric Society Monograph Series, New York, NY) 156-246.

第二篇： 

作者：Heckman, J. and R. Robb

题目：Alternative methods for evaluating the impact of interventions: an overview

来源：Journal of Econometrics, 1985, 30(1-2):239-267.

[此贴子已经被作者于2008-12-29 0:18:09编辑过]