t the most detailed level (5- digit), they have to be aggregated to ensure comparability between trading partners. Therefore the 4- digit (or sub- group) level of the Standard International Trade Classification, Revision 2 (SITC Rev. 2) serves as the basic level of comparison. Countries using the 5 digit classification tend to do so in accordance with their own statistical needs. For example, a country may not have enough trade in a particular commodity to justify giving it a separate code. Instead, that commodity may be combined with a similar product and classified to a different category.
             A further source of inconsistency between countries' reports to the U.N. stems from the use of different versions of the SITC. If a country reports its trade using Revision 1 of the SITC, the data are converted to Revision 2. Moreover, at the end of the procedure, all commodity data are further adjusted to conform to the Canadian version of the SITC at the 4- digit level. The 4- digit level of the SITC sacrifices some detail, but allows comparability between countries that use the many different national versions of the SITC.
             An example of the practices followed to serve national and international requirements is provided by the Canadian case. Until 1988, Canada used the Export Commodity Classification (XCC) to record its exports, so that the data collected had to be reclassified to the SITC to meet U.N. reporting requirements. However, the version of the SITC Canada used to submit its figures differed from the strict U.N. definition for many of the classification codes because the XCC did not convert directly to the SITC; this meant that there was not a one- to- one correspondence between the XCC and the SITC categories. Therefore, Canada aggregated the SITC codes into a "composite" SITC that was not as detailed as the international SITC at the 5- digit level.
             Some countries include electricity in their trade figures, others do not. In the...