Re:81% of 'suspects' flagged by Met's police facial recognit...
by ska on Friday July 05, 2019 @05:58PM
(#20366220)
It doesn't really work that way. Or, at least, you need some more information.
There are two big assumptions you haven't made explicit. One, that the FN rate is low. If it isn't you haven't necessarily improved your SNR at all, you're just spent resources looking in the wrong place. Two, that the cost of a FP is 0. The latter obviously isn't true, so you have to weight the positive and negative impacts across all scenarios before you can tell if this is a net win.
Think of this in a different context; say I have a new cancer screening protocol, with a low FN but also a high FP rate. So everyone we mark as positive gets a follow up biopsy. You say "great, we catch x% more cancers now than the old way, good win". Until someone points out that biopsies have a y% rate of catastrophic infection resulting in death. Depending how these rates balance out, we may be killing more people that we are saving....
There are two big assumptions you haven't made explicit. One, that the FN rate is low. If it isn't you haven't necessarily improved your SNR at all, you're just spent resources looking in the wrong place. Two, that the cost of a FP is 0. The latter obviously isn't true, so you have to weight the positive and negative impacts across all scenarios before you can tell if this is a net win.
Think of this in a different context; say I have a new cancer screening protocol, with a low FN but also a high FP rate. So everyone we mark as positive gets a follow up biopsy. You say "great, we catch x% more cancers now than the old way, good win". Until someone points out that biopsies have a y% rate of catastrophic infection resulting in death. Depending how these rates balance out, we may be killing more people that we are saving....