C.4. Bilateral or “mirror” data
10.67. The exchange of statistics between countries, especially main partners, is seen as a particularly relevant exercise for compilers of statistics on the international supply of services. However, bilateral or “mirror” statistics should not be seen as a major data source on their own, but should rather serve as a reference for checking and adjusting collected data. It should be noted that because of differences in applied concepts and data collection/compilation procedures, such mirror data should be consulted with caution and only after the necessary reconciliation. It is also well known that mirror statistics on the international supply of services often contain asymmetries, owing at least in part to coverage, sampling and methodological differences. An investigation into bilateral asymmetries between the United States and Canada is presented below.
10.68. Mirror data is a potentially useful means of collecting information on mode 4 number of persons since sending (exporting) countries may be more easily able to collect information for some categories, especially contractual-service suppliers and those travelling for negotiation purposes, than for others, such as intracorporate movements or self-employed migrants. The "receiving" (importing) countries may have such information as well.
 More information on potential sources for mode 4, and by direction of movement (ingoing/outgoing), is provided in table 11.8.