The obscure maths theorem that governs the reliability of Covid testing

There's been much debate about lateral flow tests - their accuracy depends on context and the theories of a 18th-century cleric

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Maths quiz. If you take a Covid test that only gives a false positive one time in every 1,000, what's the chance that you've actually got Covid? Surely it's 99.9%, right?

No! The correct answer is: you have no idea. You don't have enough information to make the judgment.