Uh, you don't know what the ex-con hasn't been caught for either. Maybe they got away with murder 10 years ago, but where only caught for their petty theft 5 years later.
Which also undermines the usefulness of judging based on criminal record...
How is that? It seems reasonable to believe, all else being equal, that someone that's been convicted of a crime has committed more crimes than someone that hasn't been convicted.
Why? Being caught committing a crime is less a sign of committing a crime but in doing it in a way that got you caught. Serial committers often go years uncaught... from petty burglars to assaulters.. a lot of people get caught their first time, its the ones who dont get caught that are more dangerous...