TY - JOUR AU - Heckman, James J AU - Urzua, Sergio TI - Comparing IV With Structural Models: What Simple IV Can and Cannot Identify JF - National Bureau of Economic Research Working Paper Series VL - No. 14706 PY - 2009 Y2 - February 2009 DO - 10.3386/w14706 UR - http://www.nber.org/papers/w14706 L1 - http://www.nber.org/papers/w14706.pdf N1 - Author contact info: James J. Heckman Department of Economics The University of Chicago 1126 E. 59th Street Chicago, IL 60637 Tel: 773/702-0634 Fax: 773/702-8490 E-Mail: jjh@uchicago.edu Sergio S. UrzĂșa Department of Economics University of Maryland 3115J Tydings Hall College Park, MD 20742 Tel: 301-405-3266 E-Mail: urzua@econ.umd.edu AB - This paper compares the economic questions addressed by instrumental variables estimators with those addressed by structural approaches. We discuss Marschak's Maxim: estimators should be selected on the basis of their ability to answer well-posed economic problems with minimal assumptions. A key identifying assumption that allows structural methods to be more informative than IV can be tested with data and does not have to be imposed. ER -