@techreport{NBERt0230,
 title = "Combining Panel Data Sets with Attrition and Refreshment Samples",
 author = "Keisuke Hirano and Guido W. Imbens and Geert Ridder and Donald B. Rubin",
 institution = "National Bureau of Economic Research",
 type = "Working Paper",
 series = "Technical Working Paper Series",
 number = "230",
 year = "1998",
 month = "April",
 URL = "http://www.nber.org/papers/t0230",
}