Monitoring changes to product classification systems is an important component of a wide range of empirical research. In this article we develop an alogorithm for concording periodic revisions to the ten-digit Harmonized System (HS) codes used by U.S. statistical agencies to categorize international trade since 1989. We use this algorithm to construct the first comprehensive concordance of HS codes over time, and show how this concordance can be extended to incorporate future revisions. We then characterize the extent of HS-code changes since 1989 and discuss how controlling for these revisions is critical for understanding the growth of U.S. trade. Lastly, we highlight the general applicability of the algorithm to other national and international product classification systems. Key words: International trade; product classification. JEL classification: F1.
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