The Tier 1 method for identifying key categories assesses the impacts of all IPCC-defined categories on the level and trend of the national emission inventory for the time-series (e.g., 1990-2008), but works independently of any quantitative uncertainty analysis.

When using a Tier 1 approach for the level, a pre-determined cumulative emissions threshold is used to identify key categories. When categories are sorted in order of decreasing emissions, those that fall at the top of the list and cumulatively account for 95 percent of emissions are considered key categories (i.e., the largest source categories). The 95 percent threshold was established based on an evaluation of several inventories, and was designed to establish a general level where the key category analysis covers 90 percent of inventory uncertainty (see Chapter 7 of the IPCC Good Practice Guidance for a complete discussion of how this threshold was established).

The Tier 1 approach for the trend uses a 95 percent contribution threshold of the cumulative contribution to the trend assessment metric, which was also designed to establish a general level where the key category analysis covers 90 percent of inventory uncertainty. The Tier 1 method is completed using a simple spreadsheet analysis based on equations for both level and trend assessments.