The prosecutor's fallacy consists of confusing two conditional probabilities: P(A|B) and P(B|A). For instance, P(A|B) could be the chance of observing the evidence if the accused is guilty, while P(B|A) is the chance that the accused is guilty given the evidence. The latter might not make sense at all, but even when it does, the two numbers need not be equal. This fallacy is related to a common misinterpretation of P-values.