A COVID-19 test gives a positive result in an infected person with a probability of 0.9 and a negative result in an uninfected person with a probability of 0.8. What is the probability that a person is not infected if two consecutive tests give a negative result.

I think you probably need the population infection rate. Otherwise, both whole population infected or nobody currently infected would both allow a double negative result of test, but the probability of non-infection would be 0 in the first case and 1 in the second.

So, you need some other piece of information.

So, you need some other piece of information.

Yeah, I agree with @lastchance, you would need to know what % of the population are actually infected.

Perhaps, the question was misphrased, and they meant to ask the probability of being tested negative twice and not being infected at the same time. Which would be 0.8*0.8. But that's a meaningless number, it doesn't tell you what your probability of being infected (or not) is.

So I think they must have missed part of the data.

Perhaps, the question was misphrased, and they meant to ask the probability of being tested negative twice and not being infected at the same time. Which would be 0.8*0.8. But that's a meaningless number, it doesn't tell you what your probability of being infected (or not) is.

So I think they must have missed part of the data.

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