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Comment by Falell | Hacker News Reader

I think these statistics are claiming '50-90% of observed positives are false', not '50-90% of all tests return false positives'. Suppose 0.1% of the population has a condition and the test for the condition has a 0.5% false positive rate. If administered blindly, most positive tests will be false positives.

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I think this is the crux of the article's argument - more people testing more frequently and "blindly" means a whole lot more positive results, most of which will be false.

Such a test would still be very useful, when combined with other independent weak tests. If 5 weak tests in a row confirm you have cancer, then you should worry.

And anyway just report the numbers to the doctor and the patient. "This test has a 5:1 likelihood ratio, so if the odds of you having the disease were 1 to 100, it's now 5 : 100 or 1 to 20."

Everyone needs to get a better grasp on probability.

Which doesn't mean it's not a good test to rule out the tested condition if there are symptoms that might indicate the condition. Sometimes it's clinically valuable to rule out a condition, particularly if the appropriate treatment for another possible condition would be drifter if the tested-for condition might be present. There are people whose training makes them, in principle, the appropriate experts to decide when these tests are needed and worthwhile, they're called physicians.

>'50-90% of observed positives are false'

if ^ is what he means, he should write that. The term "false-positive" has a very clear definition in medical statistics.

Fair enough. But the more rare a condition, the more observed positives will be false.

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