At its heart, the fallacy involves assuming that the prior likelihood of a random match is equal to the likelihood that the defendant is innocent. For instance, if a perpetrator were known to have the same blood type as a defendant and 10% of the population to share that blood type, then one version of the prosecutor's fallacy would be to claim that on that basis alone the likelihood of the defendant's guilt is 90%. The actual likelihood would depend on the size of the population with a matching blood type and would likely be much lower.