A note on using Comtrade for empirical trade research

International trade statistics reported in accordance with the Harmonized System (HS) are difficult to compare across time and countries because of frequent amendments to its product classification. To ease this problem, the United Nations Statistics Division (UNSD) converts national trade statistics into older HS classifications and makes these recompiled statistics available through its Comtrade database. This note reviews the nature of HS amendments and the UNSD data conversion method, and considers the circumstances under which recompiled statistics may not serve as a good substitute for original statistics.