In early October 2022, the FBI released its long-awaited compilation of 2021 crime data. But this data differed sharply in content and quality from previous years due to a transition in the way the government collects crime data. Specifically, 2021 was the first year to rely exclusively on a recently updated system for tracking crime data, the National Incident-Based Reporting System. Many agencies were not able to transition to the new format in time. As a result, the bureau received full-year reports from agencies covering just half of the country’s population. By comparison, earlier reports included a full year of data from agencies covering roughly 95 percent of the population.

To fill these gaps, the FBI’s report on national crime trends relied heavily on estimates. The agency estimated crime trends for 2021 based on the data it had available. Then it went back to 2020 and applied that same estimating methodology as if that year’s data had been similarly incomplete. In doing so, the bureau aimed to create an appropriate “apples to apples” comparison, despite the differences in data quality. The FBI also used upper- and lower-bound estimates given the uncertainty about the agency’s conclusions, estimates we represent as a rough margin of error.