Perhaps they can't get statistical significance. You'd ballpark a 0.5% infection rate in the Bay Area going by deaths and current best guesses for IFR. Your false positive rate might exceed that.
These tests are better done in highly infected areas like NYC.
We have randomized PCR studies and ships where everyone got tested. Yes, there is error there (false negatives), but there's an upper bound to how much there can be.
These tests are better done in highly infected areas like NYC.