# SafeGraph Mobility

• Source name: safegraph
• First issued: 23 June 2020
• Number of data revisions since 23 June 2020: 0
• Date of last change: never
• Available for: county, state (see geography coding docs)

This data source uses data reported by SafeGraph using anonymized location data from mobile phones. SafeGraph provides social distancing metrics to eligible researchers who obtain an API key. SafeGraph provides this data for individual census block groups, using differential privacy to protect the privacy of individual people in the data.

Delphi creates features of the Safegraph data at the census block group level, then aggregates these features to the county and state levels. The aggregated data is freely available through the COVIDcast API.

For precise definitions of the quantities below, consult the SafeGraph social distancing metric documentation.

Signal Description
completely_home_prop The fraction of mobile devices that did not leave the immediate area of their home (SafeGraph’s completely_home_device_count / device_count)
full_time_work_prop The fraction of mobile devices that spent more than 6 hours at a location other than their home during the daytime (SafeGraph’s full_time_work_behavior_devices / device_count)
part_time_work_prop The fraction of devices that spent between 3 and 6 hours at a location other than their home during the daytime (SafeGraph’s part_time_work_behavior_devices / device_count)
median_home_dwell_time The median time spent at home for all devices at this location for this time period, in minutes

After computing each metric on the census block group (CBG) level, we aggregate to the county-level by taking the mean over CBGs in a county to obtain the value and taking sd / sqrt(n) for the standard error, where sd is the standard deviation over the metric values and n is the number of CBGs in the county. In doing so, we make the simplifying assumption that each CBG contributes an iid observation to the county-level distribution. n also serves as the sample size. The same method is used for aggregation to states.

## Lag

SafeGraph provides this data with a three-day lag, meaning estimates for a specific day are only available three days later. It may take up to an additional day for SafeGraph’s data to be ingested into the COVIDcast API.