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How can a confidence interval possibly be calculated without accounting for the number of tests—not just the number of positive tests—and the total population s
by zaroth 7y ago
How can a confidence interval possibly be calculated without accounting for the number of tests—not just the number of positive tests—and the total population size?
What TFA shows us is the Rt assuming that all cases are known.
The model which knows nothing about testing rate, cannot possibly be used to predict R0 or Rt.
As a simple proof, consider what the model would output I f testing simply stopped. 0 out of 0 tests positive. After several days, the model outputs an Rt=0 with 100% confidence.
If you could account for testing rate, and then attempt to account for missed cases through proxies using hospitalization rates, ILI surveillance data, excess fatality, as well as incubation period, and an asymptomatic rate... maybe then you could draw error bars around an estimate of Rt that would pass the smell test?
If testing was run against 1,000 randomly selected people per geographical area each day, with a positivity rate reported over time, that would provide the perfect basis for making these calculations. The time series of test results that we actually have today require enormous amounts of unpacking before they can be used safely to extrapolate much of anything.
- richk449 7y agoRt is based on the rate of change, not the absolute value, right? So if testing coverage is constant in time, then doesn’t it drop out? Of course, it isn’t constant with time, but it is also probably not changing that drastically over short time frames (like the seven days the author uses), so it shouldn’t throw the results off too much.
- zaroth 7y agoEven if the number of tests stay the same, but just the testing criteria shift over time, it would invalidate the daily positive test count for use in determining Rt without somehow adjusting for the resulting bias.
- lolc 7y agoAt first I was happy to see a confidence interval in the plot. Then I realized the plots are based on the same numbers anybody else uses: new cases. I'm tired of people stretching the available numbers too thin. There is only so much information we can get out of them. And yes like you say I hope we will see daily sampling soon. As tests are geared up that should be easy enough. As an aside: The RKI in Germany is collecting weekly samples to track the flu. They have been testing for Corona now for eight weeks. In those tests, they found 11 positive samples out of 1089. I don't know the reliability of the tests though. The numbers are certainly too small to estimate noise after eight weeks.