@techreport{NBERw14837,
 title = "Concording U.S. Harmonized System Categories Over Time",
 author = "Justin R. Pierce and Peter K. Schott",
 institution = "National Bureau of Economic Research",
 type = "Working Paper",
 series = "Working Paper Series",
 number = "14837",
 year = "2009",
 month = "April",
 URL = "http://www.nber.org/papers/w14837",
 abstract = {This paper: outlines an algorithm for concording U.S. ten-digit Harmonized System export and import codes over time; describes the concordances we construct for 1989 to 2004; and provides Stata code that can be used to construct similar concordances for arbitrary beginning and ending years from 1989 to 2007.},
}