Testing and case data for the tests-per-case analysis is as of Dec. 31, 2020, and based on the residence of the person tested. Up to 6 percent of cases and 9 percent of tests are missing test takers’ geographic information in any given county. Demographic data for Bay Area neighborhoods is from the U.S. Census Bureau’s 2018 5-year American Community Survey.

Since late July, all labs testing for COVID-19 in California are required to collect and report race and ethnicity data. Despite that, according to the Department of Public Health, in the 30 days ending Feb. 22, 37 percent of tests and 14 percent of cases statewide lacked that information, making analysis of testing by ethnicity an unreliable measure.