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Date Coding and Revisions

Every observation in the COVIDcast Epidata API has two dates attached:

  • time_value: The time the underlying events happened. For example, when a data source reports on COVID test results, the time value is the date the results were recorded by the testing provider.
  • issue: The date the estimates were issued. For example, a COVID test result might be recorded on October 1st, but it may take several days for that report to be collected, aggregated, received by Delphi, and added to our database. The issue date is when Delphi makes the data available.

For example, consider using our doctor visits signal, which estimates the percentage of outpatient doctor visits that are COVID-related, and consider a result row with time_value = "2020-05-01" for geo_value = "pa". This is an estimate for the percentage in Pennsylvania on May 1, 2020, which was issued on May 5, 2020. The delay is due to the aggregation of data by our source and the time taken by the API to ingest the data provided. Later, the estimate for May 1st could be updated, perhaps because additional visit data from May 1st arrived at our source and was reported to us. This constitutes a new issue of the data, and would be reported with a new issue date.

The format of the time_value and issue dates depends on the time_type API parameter. Each data source is available for specified time types; check each source’s documentation for details on supported time types.

The available time types include:

  • day: A daily observation. The time_value and issue are both reported with year, month, and day in YYYYMMDD format. (The API clients convert this into convenient date objects.)
  • week: A weekly observation, recording events over 7 days. The time_value and issue are reported with a year and a week number ranging from 1 to 53, in YYYYWW format. These weeks are MMWR weeks as defined by the National Notifiable Diseases Surveillance System, also known as “epiweeks”. (The API clients convert these into date objects representing the first day of the MMWR week, and for those not using the clients, packages are available to convert MMWR weeks in many common programming languages.)