Let's say there is some crime committed and that only 5% of similar crimes are committed by someone like Person A (based on demographics, personality type, previous criminal record, etc.). If the police later find evidence suggesting that Person A is the perpetrator of a crime and that there is only a 10% chance that the evidence could exist if Person A is innocent, then does that mean there is a 90% chance that Person A is guilty? Or do we have to factor in the fact that there was only a 5% probability that A was guilty before the evidence was found? Thanks!
What we're trying to get to is the probability, given all the evidence, that A is guilty. Let H be the hypothesis that A is guilty. You're supposing that our initial probability for H is 5%, i.e., .p(H) = .05. Then we get a piece of evidence – call it E – and the probability of E assuming that H is false is 10%, i.e., p(E/not-H) = .1. Your question: in light of E, how likely is H? What's p(H/E)? We can't tell. We need another number: p(E/H). We need to know how likely the evidence is if A is guilty. And we can't infer that from p(E/not-H). Why not? Well, suppose the evidence is that the Oracle picked A's name out of a hat with 10 names, only one of which was A's. The chance of that if A is not guilty is 10%, but so is the chance if A is guilty (assuming Oracles don't really have special powers.) iI this case, the "evidence" is actually irrelevant. The crucial question is this: what's the ratio of p(E/H) to p(E/not-H)? Intuitively, does H do a good job of explaining E? And knowing only one...
- Log in to post comments