@techreport{Ruggles2008,
author = {Ruggles, S. and Sobek, M. and Alexander, T. and Fitch, C.A. and Goeken, R. and Hall, P.K. and King, M. and Ronnander, C.},
title= {Integrated Public Use Microdata Series: Version 4.0 [Machine-readable database]},
year = {2008},
publisher = {Minnesota Population Center},
}