Those interested in the JEP symposium linked below but would like something a bit more technical should look at Heckman's recent NBER paper:
Building Bridges Between Structural and Program Evaluation Approaches to Evaluating Policy
J J Heckman , NBER 16110, June
This paper compares the structural approach to economic policy analysis with the program evaluation approach. It offers a third way to do policy analysis that combines the best features of both approaches. We illustrate the value of this alternative approach by making the implicit economics of LATE explicit, thereby extending the interpretability and range of policy questions that LATE can answer.