For every criminal found that means 4 innocent people are flagged. Hence the 80% rate. But if there are 1 in 10,000 people which are criminals, then 5 total p

For every criminal found that means 4 innocent people are flagged. Hence the 80% rate. But if there are 1 in 10,000 people which are criminals, then 5 total people out of 10,000 will be flagged. So the chance of being flagged would be 5 in 10,000 or 1 in 2,000 which would be about a .05% false positive rate overall. I think everyone forgets about in a situation like this is the false negative rate -- or missing a flagged person. If they miss 90% of the people they flag, then the system is pretty much worthless.