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There is no "typical" I'm afraid. Whether a test is useful or not depends on error rates, priors, and application of the result. For example, consider this sce

by DenisM 7y ago

There is no "typical" I'm afraid. Whether a test is useful or not depends on error rates, priors, and application of the result. For example, consider this scenario: - Virus has R0 of 2.5 - the test for the current infection (i.e. not antibody) is cheap and abundant - the test has 20% false positives - the test has 20% false negatives - Application: test every single person and confine everyone who tested positive to their homes, releasing the rest. Repeat weekly. Take no other action. Result #1: 20% of the population are in isolation. 20% of your economy is stalled, rather than 100%. Result #2: 1 in 5 infected will go undetected. Still 4 out of 5 are isolated, reducing the number of infected out there by a factor of 5, therefore your R0 drops from 2.5 to 0.5 The number of infected people on the street gets cut in half every week. Epidemic fizzles out. So even low-precision tests are useful. What is useless is tests that do not have a reliable metric for false positive / negative. Which is what many of the AB tests are.

perl4ever 7y ago
"Result #1: 20% of the population are in isolation. 20% of your economy is stalled, rather than 100%." This bugs me, because in this hypothetical, you're implying something as a point of comparison which is nothing like reality. 100% of the population is not in isolation - the only people who are in isolation are those who have had such severe symptoms they've been tested and it came back positive, or they're really obviously linked to such a person. And obviously 100% of the economy is not stalled; it may not be precisely clear what the impact is so far, but people are talking about single digit GDP percentage point declines. If you frame that as the worst economic news since the great recession, or put it in context with the job losses, it's pretty bad, but it clearly could be ten times worse and it's not. I get that maybe you meant it as just an example and specifics aren't important to your point about statistics, but I feel like I've read similar posts to yours that kind of get the nonsensical scenario out there, that we've shut down "100%" to where people kind of get used to it even though it's not reality.
- Retric 7y ago
  US’s isolation started very late in Q1. It requires a very large decline to average out to ‘just’ 1%.
  - perl4ever 7y ago
    I don't feel that makes a difference to what I just wrote. But I can do some very rough arithmetic along those lines to see what the decline would have to be. The GDP figure just reported is -4.8%. Based on what I recall, the inflection point was about halfway through March. Friday the 13th is when the state government in NY finally said anyone who can, work from home. That's 0.5/3 months or ~17% of the quarter I'd assign to "shutdown". 0.83 * 1 + .17 * x = .952 ...I don't trust myself to do high school algebra, so I asked Wolfram Alpha and it said x = 72%. Ok, so the reported figure of nearly -5% probably equates to a little over a quarter of the economy missing. I don't think I need to concede my point about a 100% stoppage not happening.
    - Retric 7y ago
      The larger point stands it’s just ‘but people are talking about single digit GDP percentage point declines‘ that I was correcting. That said, Pennsylvania and several other states waited until April to order a shutdown, but the observed GDP decline was annualizing a ~1% though there was growth in the ready of Q1. Long story short, that 4.8% is absolutely worthless.
      - perl4ever 7y ago
        The bit you quote from me seems strictly accurate; by single digit I mean < 10%. That is what the news is reporting. It is factual, and my inference that the economy is not shut down hasn't been questioned. Saying that the difference is larger when you consider only part of the quarter is correct too, but 4.8% is not "absolutely worthless", it just needs to be interpreted properly. As in my previous comment, you can approximately derive the actual amount of decline, which is a fraction of a complete shutdown.
        7y ago
jxramos 7y ago
So interesting, so even with inaccurate tests so long as we're dealing with reliable metrics backing those tests up then the degree of actual accuracy that intersects with the nominal accuracy is enough to carry out the compounding with and effect targeted quarantines with. Probability products stack up quick. Hope that makes some bit of sense.

by DenisM 7y ago

perl4ever 7y ago

"Result #1: 20% of the population are in isolation. 20% of your economy is stalled, rather than 100%." This bugs me, because in this hypothetical, you're implying something as a point of comparison which is nothing like reality. 100% of the population is not in isolation - the only people who are in isolation are those who have had such severe symptoms they've been tested and it came back positive, or they're really obviously linked to such a person. And obviously 100% of the economy is not stalled; it may not be precisely clear what the impact is so far, but people are talking about single digit GDP percentage point declines. If you frame that as the worst economic news since the great recession, or put it in context with the job losses, it's pretty bad, but it clearly could be ten times worse and it's not. I get that maybe you meant it as just an example and specifics aren't important to your point about statistics, but I feel like I've read similar posts to yours that kind of get the nonsensical scenario out there, that we've shut down "100%" to where people kind of get used to it even though it's not reality.

Retric 7y ago
US’s isolation started very late in Q1. It requires a very large decline to average out to ‘just’ 1%.
- perl4ever 7y ago
  I don't feel that makes a difference to what I just wrote. But I can do some very rough arithmetic along those lines to see what the decline would have to be. The GDP figure just reported is -4.8%. Based on what I recall, the inflection point was about halfway through March. Friday the 13th is when the state government in NY finally said anyone who can, work from home. That's 0.5/3 months or ~17% of the quarter I'd assign to "shutdown". 0.83 * 1 + .17 * x = .952 ...I don't trust myself to do high school algebra, so I asked Wolfram Alpha and it said x = 72%. Ok, so the reported figure of nearly -5% probably equates to a little over a quarter of the economy missing. I don't think I need to concede my point about a 100% stoppage not happening.
  - Retric 7y ago
    The larger point stands it’s just ‘but people are talking about single digit GDP percentage point declines‘ that I was correcting. That said, Pennsylvania and several other states waited until April to order a shutdown, but the observed GDP decline was annualizing a ~1% though there was growth in the ready of Q1. Long story short, that 4.8% is absolutely worthless.
    - perl4ever 7y ago
      The bit you quote from me seems strictly accurate; by single digit I mean < 10%. That is what the news is reporting. It is factual, and my inference that the economy is not shut down hasn't been questioned. Saying that the difference is larger when you consider only part of the quarter is correct too, but 4.8% is not "absolutely worthless", it just needs to be interpreted properly. As in my previous comment, you can approximately derive the actual amount of decline, which is a fraction of a complete shutdown.
      - 7y ago

jxramos 7y ago

So interesting, so even with inaccurate tests so long as we're dealing with reliable metrics backing those tests up then the degree of actual accuracy that intersects with the nominal accuracy is enough to carry out the compounding with and effect targeted quarantines with. Probability products stack up quick. Hope that makes some bit of sense.