Q: Why does the sum of monthly trade data sometimes differ from the reported annual total in UN Comtrade?
A: There are several valid reasons why monthly and annual trade data may not align perfectly:

• Separate Collection and Processing:
  Monthly and annual data are compiled and processed independently. Rules applied to estimate missing quantities or other attributes may vary between the two, resulting in differences.
• Data Revisions:
  Countries regularly revise their data. These revisions may be reflected in annual datasets but may not be reflected in the monthly submissions sent to UN Comtrade. Hence, annual data may be more up-to-date.
• Annual-Only Revisions:
  For practical reasons, some data revisions (e.g., shuttle trade adjustments) are made exclusively at the annual level and are not applied to the monthly data.
• Confidentiality Rules:
  Confidentiality is handled differently for monthly and annual data. For example, a single exporter in one month may trigger suppression due to confidentiality, whereas aggregated annual data with multiple exporters may not require such suppression.
• HS Classification Changes:
  If a country switches to a new Harmonized System (HS) edition partway through the year, monthly and annual data may use different classification schemes, leading to discrepancies.

These differences are normal and reflect the nature of data processing, revisions, and confidentiality handling in international trade statistics.