A key source or sink category is defined as a “[category] that is prioritized within the national inventory system because its estimate has a significant influence on a Party’s total inventory of direct greenhouse gases in terms of the absolute level of emissions, the trend in emissions, or both.” By definition, key categories have the greatest contribution to the absolute overall level of national emissions and removals.

In addition, when an entire time series of estimates is prepared, a thorough investigation of key categories must also include accounting for the influence of trends of individual categories on the trend of the Party’s total emissions and removals. This trend assessment identifies categories for which significant uncertainty in the estimate would have considerable effects on overall emission and removal trends, and therefore identifies categories that diverge from the overall trend in national emissions and removals.

Finally, a qualitative evaluation of key categories should be performed, in order to capture any key categories that were not identified in either of the quantitative analyses.

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