@techreport{NBERt0241,
 title = "When to Control for Covariates?  Panel-Asymptotic Results for Estimates of Treatment Effects",
 author = "Joshua D. Angrist and Jinyong Hahn",
 institution = "National Bureau of Economic Research",
 type = "Working Paper",
 series = "Technical Working Paper Series",
 number = "241",
 year = "1999",
 month = "May",
 URL = "http://www.nber.org/papers/t0241",
}