zlacker

This is a classic example of the false positive rate fallacy.

Let's say that there are a million people, and the police have photos of 100,000 of them. A crime is committed, and they pull the surveillance of it, and match against their database. They have a funky image matching system that has a false positive rate of 1 in 100,000 people, which is way more accurate than I think facial recognition systems are right now, but let's just roll with it. Of course, on average, this system will produce one positive hit per search. So, the police roll up to that person's home and arrest them.

Then, in court, they get to argue that their system has a 1 in 100,000 false positive rate, so there is a chance of 1 in 100,000 that this person is innocent.

Wrong!

There are ten people in the population of 1 million that the software would comfortably produce a positive hit for. They can't all be the culprit. The chance isn't 1 in 100,000 that the person is innocent - it is in fact at least 9 out of 10 that they are innocent. This person just happens to be the one person out of the ten that would match that had the bad luck to be stored in the police database. Nothing more.

>>mnw21c+(OP)
There's a good book called "The Drunkards Walk", that describes a woman who was jailed after having 2 children die from SIDS. They argued that the odds of this happening is 1 in a million (or something like that), so probably the woman is a baby killer. The prosecution had statisticians argue this. The woman was found guilty.

She later won on appeal in part because the defense showed that the testimony and argument of the original statisticians were wrong.

This stuff is so easy to get wrong. A little knowledge of statistics can be dangerous.

>>mnw21c+(OP)
Definitely they should have everyone's 3d image in the system. DNA too.

>>mnw21c+(OP)
See also: Privileging the hypothesis.

If I'm searching for a murderer in a town of 1000, it takes about 10 independent bits of evidence to get the right one. And when I charge someone, I must already have the vast majority of that evidence. To say "oh well we don't know that it wasn't Mr. or Mrs. Doe, let's bring them in" is itself a breach of the Does' rights. I'm ignoring 9 of the 10 bits of evidence!

Using a low-accuracy facial recognition system and a low-accountability lineup procedure to elevate some random man who did nothing wrong from presumed-innocent to 1-in-6 to prime suspect, without having the necessary amount of evidence, is committing the exact same error and is nearly as egregious as pulling a random civilian out of a hat and charging them.

>>Button+fJ
And even if the original stats were right. A 1 in a million event happens to about 100 people per day in the US.

>>Polyla+re1
> A 1 in a million event happens to about 100 people per day in the US.

This is a meaningless statement, you could choose literally any number for this statement, because you are missing the denominator.

>>Polyla+re1
Sure.. But the case being discussed has a maximum frequency of 1 in a million every 18 (2 terms of childbirth) months, further reduced by needing to be a woman of reproductive age, fertile, etc etc.

This case of "one in a million" does not happen frequently.

>>mnw21c+(OP)
In the legal world it apparently goes by the moniker "Prosecutor's Fallacy":

https://en.wikipedia.org/wiki/Prosecutor%27s_fallacy

>>mnw21c+(OP)
See also: the paper "Why Most Published Research Findings Are False". Previously on HN: https://news.ycombinator.com/item?id=1825007