@techreport{NBERt0136,
 title = "Identification of Causal Effects Using Instrumental Variables",
 author = "J.D. Angrist and Guido W. Imbens and D.B. Rubin",
 institution = "National Bureau of Economic Research",
 type = "Working Paper",
 series = "Technical Working Paper Series",
 number = "136",
 year = "1993",
 month = "June",
 URL = "http://www.nber.org/papers/t0136",
 abstract = {},
}