Reply to post: Re: Surely though

Zero arrests, 2 correct matches, no criminals: London cops' facial recog tech slammed

ibmalone Silver badge

Re: Surely though

You don’t understand Math, do you?

98% of false positives is when from 10,000 you select 100, 2 of whom are criminals.

This is a bit of a wriggle, since you've left out a crucial word. The term used throughout the article is false positive rate, which doesn't mean that at all. To work out the false positive rate you'd need to know the proportion of "non-criminals" in that 10,000 (or, more practically, number of people in not in the target database, otherwise we get into questions of just what makes a criminal).

The "Face Off" campaign website shows something different to the article, a nice pie chart with 2 true positive matches and 101 false positive matches for the metropolitan police. That's a false detection rate of 98%, not a false positive rate. If we assume they got all the criminals and looked at 10000 people then their false positive rate was 1%, specificity 99%. Which sounds great doesn't it? It would be really good. And yet, hypothetical sensitivity 100%, specificity 99%, still gives 98% false detection rate, because you're looking for rare events. This is really important to be aware of when applying detection methods. If you're planning on stopping someone and questioning them on the basis that they might look a bit like someone you're after, not because they were doing something suspicious or any other reason, then you are going to end up doing it to a lot of innocent people.

Looking at their breakdown for events it was used at is interesting, Remembrance Day 1 TP, 6FP (much better than Notting Hill), Notting Hill '17 1 TP 95 FP (so closer to 99% FDR), Notting Hill '16 0 TP "?" FP.

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