I was ranking "randomly selected" higher than "people volunteering" (which was one of the main issues with the santa clara study, mixed with the test used being

I was ranking "randomly selected" higher than "people volunteering" (which was one of the main issues with the santa clara study, mixed with the test used being so imprecise the error bars were big enough to include a 100% false positive rate. That is by far one of the worst designed studies I've looked at, even if it has been peer reviewed) I'm not sure the blood sampling would skew the direction you think it does, healthy individual could just as well mean that they're more likely to have had it since they're less likely to sit at home and stick to a quarantine. But you are right, it definitely counts as a sampling issue and I'll concede that ranking by quality is a lot less clear cut, I should not have been so confident