Californian antibody test finds only 1.5% of self selecting group in highest risk county actually had coronavirus

“Coronavirus may be far more widespread than known”. Or not.

Yet another small non-random study shows “48,000″ people in Santa Clara County had Coronavirus and didn’t know it, but all the study really shows may be motivated reasoning.

The Santa Clara study looked at the county with the highest number of Covid cases in California, then advertised on Facebook for people to come forward for an unvalidated test, after which the results were adjusted upwards and converted into headline grabbing ratios and extrapolated to include the whole county and to calculate case fatality rates.

Advertising for participants creates an obvious selection bias straight away – people who thought they may have had coronavirus are more likely to want to go and get tested. But people who knew they didn’t have it (because they had self isolated) might not want to turn up and stand in a queue or even catch coronavirus while they waited.

Basically, they found 50 people out of 3,330 tested positive. About half of which were likely to be false positives. They weighted the sample by zip code, race and sex, but for some reason, didn’t adjust for age, which is a defining characteristic of infection […]