Posted by Shyaku on July 12, 2020, 9:42 pm, in reply to "Can someone explain the Covid-19 stats to me?"
(1) Greater total # of tests -> greater total number of positives. This seems to be already normalized out in your numbers per 100,000 |
(2) boro-to-boro variation: Not sure why the test itself would be different from boro-to-boro, but yes, its administration might be. Swabbing vs sputum is a thing, and self-administered swab vs. nurse administered might be. Army test I know of is self-administered. A family of three I know came back inconclusive for this, then subsequent nurse-administered was negative for same three people. So it may depend how they count the inconclusives, for example and whether re-tests coming back negative are then knocked out. Maybe this is a real issue for crowded test centers like a city center, that may use army and self-administering for shortage of staff.
(3) How is testing accuracy really assessed? It is probably initially assessed in a laboratory by mixing various known amounts of virus with various known amounts of other viruses and other contaminants. For clinical use, you probably don't know accuracy because there is no independent, absolute measurement to use as a yardstick for a positive, other than taking replicate samples from the same people and comparing the replicates, and also seeing how many of those go on to acquire specific symptoms as in, say, a chest X-ray.
Also, have enough tests have been done clinically, to get a reasonable number for accuracy by taking replicates? If testing, say, the same 10,000 people twice, then if one time you get 1 positive and the next time 3 positives, you probably have a range of positives per 100,000 that encompasses both your numbers. Here, the negative rate may be considered 99.9% accurate, but the positive rate maybe only 30%. So you need quite a large amount of clinical use, I think, to refine the numbers for accuracy and maybe tweak the test. But this is mabe less of an issue than #2 (self- vs. nurse-).
Bottom line: If early/rapid response is your best weapon, and relatives of dead people screaming in your face is your worst nightmare, you are going to be very sensitive to the numbers however preliminary.