  This document describes the aggregation and anonymization process applied to
the initial version of Google COVID-19 Community Mobility Reports (published at
http://google.com/covid19/mobility on April 2, 2020), a publicly available
resource intended to help public health authorities understand what has changed
in response to work-from-home, shelter-in-place, and other recommended policies
aimed at flattening the curve of the COVID-19 pandemic. Our anonymization
process is designed to ensure that no personal data, including an individual's
location, movement, or contacts, can be derived from the resulting metrics.
  The high-level description of the procedure is as follows: we first generate
a set of anonymized metrics from the data of Google users who opted in to
Location History. Then, we compute percentage changes of these metrics from a
baseline based on the historical part of the anonymized metrics. We then
discard a subset which does not meet our bar for statistical reliability, and
release the rest publicly in a format that compares the result to the private
baseline.
