When you express things in terms of probability percentages and you offer more than one base rate e.g. Heroin addicts make up 0.01% of the population and 100% have fresh needle marks. There’s also a 0.19% chance that a non-heroin addict has fresh needle marks. What’s the probability that a randomly selected person with fresh needle marks is addicted to heroin? The answer, and a better way to ask the same question after the jump.
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