5 ms·

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

by acchow 7y ago

DeonPenny 7y ago
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.
- Retric 7y ago
  It’s normal to randomly sample multiple locations. Random samples from even 10 random locations are much more useful.
  - umanwizard 7y ago
    Not really... more data can't fix sampling bias.
    - Retric 7y ago
      It’s not a question of ‘more data’ as I could take 50 or 50,000 samples from a single person and I gain nothing about the overall population. Similarly if I sample 50 or 50,000 people from a single city I learn nothing about people outside of that city. But, by sampling N random locations I at worst have N samples to work with. PS: At the extreme, if I sample every single member of a population then ‘more data’ solved any bias problems. Smaller samples are subject to a wider range of biases.
  - DeonPenny 7y ago
    Not if you are trying to figure out the best decision for a specific city/state
- umanwizard 7y ago
  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.
  - Waterluvian 7y ago
    You're more likely to attract people of lower incomes, which we already suspect are more vulnerable to Covid given the kinds of jobs they have.
- oefrha 7y ago
  > Your never going to have a completely random test. You are not, but on the full spectrum of somewhat random selection methods, talking to people on the street is about as biased as you can get.
arenaninja 7y ago
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
- retsibsi 7y ago
  If anyone has good data on the asymptomatic proportion, I'd appreciate a link. I keep seeing 50% (or sometimes 'up to 50%'), but to the extent that I've looked at actual sources, it seems that people are conflating presymptomatic positive tests with lastingly asymptomatic infections.
- spion 7y ago
  Based on several studies using different methods, the death rate (infection fatality rate, or IFR) is likely around 0.5% 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/ 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/zwischenergebnis_covid19_case_study_gangelt_0.pdf 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 https://www.ncbi.nlm.nih.gov/pubmed/32234121 (Diamond princess ship, 0.5%) [4]: https://www.medrxiv.org/content/10.1101/2020.04.14.20062463v1 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
  - Retric 7y ago
    Dimond Princess had 14 deaths out of ~700 infected or a 2% fatality rate though that number has slowly climbed over time. As they tested everyone initially finding a large number of asymptotic cases that’s likely extremely accurate. The difference is the population was more heathy than the general population, and while older on average, it had a lower percentage of people 95+ years old.
fhsm 7y ago
“Public health experts already knew Chelsea had the state’s highest rate of confirmed COVID-19 cases and that the actual rate was probably higher” Should be 1/3 + in town with highest confirmed case rate. It may be a random sample within that town but it’s explicitly not representative of MA
animalCrax0rz 7y ago
In addition, the wording "test positive for Covid antibodies" may lead people to think that 30% of the population in MA is immune to Covid19. It obfuscates the fact that the mere presence of antibodies does not imply immunity. There has to be a sufficient amount of antibodies in order to successfully fight off reinfection (which could be worse than the initial infection) ...
dang 7y ago
Ok, changed now. Submitted title was '30% of random sample test positive for Covid antibodies in Massachusetts'

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

by acchow 7y ago

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

DeonPenny 7y ago

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.

Retric 7y ago
It’s normal to randomly sample multiple locations. Random samples from even 10 random locations are much more useful.
- umanwizard 7y ago
  Not really... more data can't fix sampling bias.
  - Retric 7y ago
    It’s not a question of ‘more data’ as I could take 50 or 50,000 samples from a single person and I gain nothing about the overall population. Similarly if I sample 50 or 50,000 people from a single city I learn nothing about people outside of that city. But, by sampling N random locations I at worst have N samples to work with. PS: At the extreme, if I sample every single member of a population then ‘more data’ solved any bias problems. Smaller samples are subject to a wider range of biases.
- DeonPenny 7y ago
  Not if you are trying to figure out the best decision for a specific city/state
umanwizard 7y ago
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.
- Waterluvian 7y ago
  You're more likely to attract people of lower incomes, which we already suspect are more vulnerable to Covid given the kinds of jobs they have.
oefrha 7y ago
> Your never going to have a completely random test. You are not, but on the full spectrum of somewhat random selection methods, talking to people on the street is about as biased as you can get.

arenaninja 7y ago

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

retsibsi 7y ago
If anyone has good data on the asymptomatic proportion, I'd appreciate a link. I keep seeing 50% (or sometimes 'up to 50%'), but to the extent that I've looked at actual sources, it seems that people are conflating presymptomatic positive tests with lastingly asymptomatic infections.
spion 7y ago
Based on several studies using different methods, the death rate (infection fatality rate, or IFR) is likely around 0.5% 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/ 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/zwischenergebnis_covid19_case_study_gangelt_0.pdf 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 https://www.ncbi.nlm.nih.gov/pubmed/32234121 (Diamond princess ship, 0.5%) [4]: https://www.medrxiv.org/content/10.1101/2020.04.14.20062463v1 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
- Retric 7y ago
  Dimond Princess had 14 deaths out of ~700 infected or a 2% fatality rate though that number has slowly climbed over time. As they tested everyone initially finding a large number of asymptotic cases that’s likely extremely accurate. The difference is the population was more heathy than the general population, and while older on average, it had a lower percentage of people 95+ years old.

fhsm 7y ago

“Public health experts already knew Chelsea had the state’s highest rate of confirmed COVID-19 cases and that the actual rate was probably higher” Should be 1/3 + in town with highest confirmed case rate. It may be a random sample within that town but it’s explicitly not representative of MA

animalCrax0rz 7y ago

In addition, the wording "test positive for Covid antibodies" may lead people to think that 30% of the population in MA is immune to Covid19. It obfuscates the fact that the mere presence of antibodies does not imply immunity. There has to be a sufficient amount of antibodies in order to successfully fight off reinfection (which could be worse than the initial infection) ...

dang 7y ago

Ok, changed now. Submitted title was '30% of random sample test positive for Covid antibodies in Massachusetts'