@techreport{NBERw12574,
 title = "Understanding Instrumental Variables in Models with Essential Heterogeneity",
 author = "James J. Heckman and Sergio Urzua and Edward J. Vytlacil",
 institution = "National Bureau of Economic Research",
 type = "Working Paper",
 series = "Working Paper Series",
 number = "12574",
 year = "2006",
 month = "October",
 URL = "http://www.nber.org/papers/w12574",
}