@techreport{NBERt0223,
 title = "Overidentification Tests with Grouped Data",
 author = "Caroline Hoxby and M. Daniele Paserman",
 institution = "National Bureau of Economic Research",
 type = "Working Paper",
 series = "Technical Working Paper Series",
 number = "223",
 year = "1998",
 month = "February",
 URL = "http://www.nber.org/papers/t0223",
 abstract = {This paper examines the validity of overidentification tests and exogeneity tests in the presence of grouped data.  We find that even a small intra-group correlation, when instruments do not vary within groups, may generate a substantial bias in the standard overidentification tests described in textbooks.},
}