San Francisco COVID-19 Data Questions

We're putting answers to common questions about data in our COVID-19 Data Tracker below.

Why are testing and cases data lagged? Why can’t I see more recent data?

Data shown on the Tracker are the most reliable and accurate data available. After a COVID-19 test specimen has been collected by a health care provider, there are two important process steps that must be completed for the City to have accurate data:
  1. Lab Processing & Reporting: Tests are sent to laboratories to process the test. There are many labs processing tests including commercial, clinical, and hospital laboratories, as well as the San Francisco Public Health Laboratory. Once a lab processes the test, they then report the result.
  2. Data Validation: Once the City receives test results data, the data must be validated and any data issues must be resolved. During this step, staff sometimes uncover gaps or problems in the data including labs not reporting properly; they then have to reach out to labs to correct the data.
Both of these steps take a significant amount of effort on the part of medical providers, laboratories, and City staff. The data shown on the tracker is the data that Department of Public Health staff have deemed reliable. If either of these processing steps is streamlined or shortened in the future, the data delay may decrease and more recent data may be shown.
Note that for positive tests, additional verification steps and interviews are completed to confirm the case is a resident of San Francisco (as described below).

Why do the number of positive COVID-19 cases change on past dates?

The San Francisco Department of Public Health (SFDPH) depends on data reported by private and public labs and providers. SFDPH staff compile these data by close of business each day and report them the following morning.
Tests and cases can increase or decrease for previous days because of lab processing time, data validation, and case investigations
Because of the high volume of tests that labs are performing, they are not always able to report on tests on the same day.  This can often lead to labs reporting the current days results, as well as results for previous days. In addition, during data validation, SFDPH sometimes identifies data gaps or delays with certain labs that must be resolved (which may result in data being added or corrected for previous days).
In addition, SFDPH follows up and completes case investigations with every positive test. During investigation, some tests are found to be for persons living outside of SF and some are duplicates of previously received results. These are subsequently removed and are not included in the total San Francisco COVID-19 case count.
How data flows from labs and providers to Public Health
Below is a diagram representing how data comes to SFDPH and is then reported. Labs or providers may submit reports to SFDPH with positive cases from earlier. SFDPH then compiles them.
The total positive cases will go up by the number of cases reported, but the days on which individuals tested positive might be earlier.
In the below example, if on 4/2 you had looked at the totals for that day, you would have seen 2 positive cases (the two cases in grey on 4/2/20).
Yet if you checked back on 4/3 you would see 3 positive cases for 4/2 due to Lab 2 reporting another case for that day (the green case on 4/2/20).
And what about media sources being different?
Media sources are often picking up from the totals posted daily on public health websites. They are often collecting data from those sites as well as other sources. Since the reported totals on the site are a single number, the media reports may not capture COVID-19 positive test result dates.

Your best bet for the most up to date information for San Francisco is the case data our Department of Public Health shares. See the dataset and COVID-19 Data Tracker reports for those numbers.

Example diagram of how data from labs is submitted to the Department of Public Health, combiled and then added to existing reports. Labs could submit data that includes positive test results from earlier than the report date.

Do you report on COVID-19 recoveries?

No, the dashboards and datasets on the COVID-19 Data Tracker do not reflect the number of COVID-19 recoveries. The San Francisco Department of Public Health (SFDPH) is focused on caring for patients, reducing the spread of COVID-19, and tracking the surge. Recoveries happen in many settings, including homes, residential facilities, and hospitals, and are not routinely reported to the local health department. This is consistent with SFDPH reporting in the regular flu season.