The authors use U.S. county data (3,058 observations) and 41 conditioning variables to study growth and convergence. Using OLS and 3SLS-IV the authors report on the full sample and metro, non-metro, and 5 regional samples.
Link
http://econpapers.repec.org/scripts/redir.pl?u=http%3A%2F%2Fwww.economics.emory.edu%2FWorking_Papers%2Fwp%2Flevy_05_29_paper.pdf;h=repec:emo:wp2003:0529