The visitdata.org website is powered by Foursquare’s US foot traffic data, derived from our panel of 13 million always-on opted-in users. Visits by that panel are normalized 1) for changes in the panel's size, and 2) to better reflect the age-gender distribution of each state. Note that the panel data, and the normalized estimated number of visits derived from the panel data, are subject to various errors and biases. Prior to May 1, 2020, the methodology was as follows:
The data is represented as an index. Foot traffic for a given county on a given day is indexed to average foot traffic for that county in the month of February 2020. Increases or decreases relative to that average are corroborated with a measure of statistical significance. In this case, we are simply testing for a difference in means, given “n” size, effect size, and variance.

To learn more about Pilgrim – Foursquare’s “snap-to-place” technology which powers our visit data – click here.
If data is missing for a particular County or Venue Type, that means there weren't enough visits for that Venue Type in that County to have usable data.
Grouped groups the different kinds of places into 25 different kinds of places, see more about how that grouping is done here. All has a much finer-grained categorization of places.
Any Venue Type in All can be seen in Foursquare's Explore page. For example, if you are looking at the data for Big Box Stores in Santa Clara county and are wondering what that means, you can look up Big Box Stores in Santa Clara county in Foursquare Explore here. The groupings of Venue Types in the Grouped view are described here.
It's here.

Each day is for the 24-hour period ending at 5pm Pacific Time on that date. For example Monday's data cover from Sunday 5pm to Monday 5pm. (Doing it this way allows us to post new data with the minimum lag.)
If you are a crisis response team who needs help working with this data — please contact support@visitdata.org. VisitData.org needs volunteer programmers, data analysts, and crisis team liaisons — please contact volunteers@visitdata.org or visit https://github.com/VisitData-org/ca_visit_tracking.