One particular criticism of the Stanford study was that they recruited via Facebook adverts, and that method may be influenced in recruitment by the "desire to get a test if you had reason to believe that you, or someone near you, had the virus"

In more detail: https://statmodeling.stat.columbia.edu/2020/04/19/fatal-flaw...

This USC study seems to have recruited differently: "Participants were recruited via a proprietary database that is representative of the county population. The database is maintained by LRW Group, a market research firm" Via: http://www.publichealth.lacounty.gov/phcommon/public/media/m...