41–50 of 77 posts
Earlier quoted context omitted.
Your never going to have a completely random test. Your randomness comes from you picking for an area and the people who chose to do the test. Saying otherwise is being pedantic to your detriment.
It’s normal to randomly sample multiple locations. Random samples from even 10 random locations are much more useful.
A third of a random sample taken in Chelsea, Massachusetts : https://en.wikipedia.org/wiki/Chelsea,_Massachusetts >Chelsea is [...] directly across the Mystic River from the city of Boston. [...] It is also the second most densely populated city in Massachusetts behind Somerville. A random sample in a highly affected and extremely dense area is not a model for the whole state.
>Nearly one third of 200 Chelsea residents who gave a drop of blood to researchers on the street this week tested positive for antibodies linked to COVID-19, a startling indication of how widespread infections have been in the densely populated city. >The doctors used a diagnostic device made by BioMedomics, of Morrisville, N.C., to analyze drops of blood. It resembled an over-the-counter pregnancy test and generated…
Because you quoted the article using BioMedomics' antibody diagnostic test, I will quote the accuracy from their site[0]: > In order to test the detection sensitivity and specificity of the COVID-19 IgG-IgM combined antibody test, blood samples were collected from COVID-19 patients from multiple hospitals and Chinese CDC laboratories. The tests were done separately at each site. A total of 525 cases were tested: 397…
Hmm, I wonder if they attempt to control for bias. It sounds as though they were asking random passersby if they wanted a COVID-19 antibody test. If so, it seems that the sample group could be biased toward including more people who suspected that they may have been infected (previously or currently) than a truly random sample.
Earlier quoted context omitted.
Specificity of 90% seems really low? Does that mean almost 1 in 10 would test positive even if nobody had it?
Yes, it's 10% false positive. But it's not "really low" in the sense that it's typical for antibody tests. Which is one reason not to do wide-spread testing.
1/10 * 1/10 * 1/10 (1/1000)
Can we change the title? Article title is "Nearly a third of 200 blood samples taken in Chelsea show exposure to coronavirus" The sample is clearly not random
Yep.. I saw a similar headline claiming LA infections are 55x the reported rate. I think the math is likely simpler, and the infection is somewhere between 3x-5x (10x tops). I've read a few reports saying about 50% of people are asymptomatic, 30% show symptoms that aren't bad enough for hospitalization and 20% require hospitalization. I can't find the source I got it from but I believe it was a Chinese study
Can we change the title? Article title is "Nearly a third of 200 blood samples taken in Chelsea show exposure to coronavirus" The sample is clearly not random
Yep.. I saw a similar headline claiming LA infections are 55x the reported rate. I think the math is likely simpler, and the infection is somewhere between 3x-5x (10x tops). I've read a few reports saying about 50% of people are asymptomatic, 30% show symptoms that aren't bad enough for hospitalization and 20% require hospitalization. I can't find the source I got it from but I believe it was a Chinese study
So if you go back the average time from infection to death (2-3 weeks) you can get a sense what the ratio was in your area back then.
[1]: https://mothership.sg/2020/03/iceland-covid-19/ (I did the work on the side, extrapolated to about 0.2%-0.5%)
[2]: https://www.land.nrw/sites/default/files/asset/document/zwis... (Germany study, 0.37%, decent sampling method)
[3]: https://www.ncbi.nlm.nih.gov/pubmed/32234121 (Diamond princess ship, 0.5%)
[4]: https://www.medrxiv.org/content/10.1101/2020.04.14.20062463v... (Santa Clara county study, 0.15% - 0.3% if you do the math). Methodological problems with the method of sampling that might increase the number of positives and the test specificity issues might increase false positives, but probably still in the ballpark
Earlier quoted context omitted.
Your never going to have a completely random test. Your randomness comes from you picking for an area and the people who chose to do the test. Saying otherwise is being pedantic to your detriment.
You can choose to offer people some incentive to take the test. If you offered $50 per person you would probably eliminate most sampling bias.
>Nearly one third of 200 Chelsea residents who gave a drop of blood to researchers on the street this week tested positive for antibodies linked to COVID-19, a startling indication of how widespread infections have been in the densely populated city. >The doctors used a diagnostic device made by BioMedomics, of Morrisville, N.C., to analyze drops of blood. It resembled an over-the-counter pregnancy test and generated…
This doesn't seem that random - it is sampling from people on the street. So it's going to miss people who aren't going out onto the street. And people who are going out are more likely to catch something than those who aren't.