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[return to "Cops say criminals use a Google Pixel with GrapheneOS – I say that's freedom"]
1. tempod+nv[view] [source] 2025-07-23 16:14:23
>>pabs3+(OP)
Criminals also go to department stores. And to hairdressers. And they drink water. There is no end to deeply suspect activities criminals engage in!
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2. tzs+1e1[view] [source] 2025-07-23 20:25:00
>>tempod+nv
Do criminals do those things much more frequently than non-criminals?

In Spain 86% of phone buyers buy from the top 5 brands. Pixel is not among those. Only 1.7% buy Pixel phones. Police say that they have observed that among criminals the percent with Pixels is much higher.

Suppose police have a dead store clerk and only 3 people who could have possibly done it, and those people are 1 Pixel owner, 1 Samsung Galaxy owner, and 1 Apple iPhone owner. Given that criminals are buying Pixels at a rate higher than the general population does, and assuming they are not buying Galaxies and iPhones at a higher rate, can the police use that in statistically valid way to help their investigation?

The answer is yes.

Let (c) = the probability that a random phone owner in Spain is a criminal.

Let (p) = the probability that a random phone owner owns a pixel, which is 0.017 in Spain.

Let (p|c) = the probability that someone owns a Pixel given that they are a phone owning criminal. Police say that this is higher than 0.017, but they do not give a number. I'd expect they wouldn't really notice if it was only a little higher. I'd guess it would need to be at least 0.05 for them to notice, so let's go with that. If someone finds a better number it is easy to adjust in the following calculations.

Let (c|p) = the probability that someone is a criminal given they have a Pixel.

Bayes' Theorem tells us that (p|c)(c) = (c|p)(p).

Rearrange that to get (c|p) = (p|c) (c) / (p). Plugging in 0.05 for (p|c) and 0.017 for (p) gives:

(c|p) = 2.9 (c)

In our case with 3 people to investigate, one with a Pixel and two without, if we are sure that one of them must be the criminal the probability that it will be the Pixel owner is 59.2%. It is 20.4% for the Galaxy owner and 20.4% for the iPhone owner [1]. If the police don't have the resources to investigate all 3 in parallel they should check out the Pixel owner first.

[1] Actually, I don't think that is quite right. I think that because I added the condition that we are sure it must be one of them the distribution will change slightly. It still should be close though.

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3. imperf+GW5[view] [source] 2025-07-25 10:51:36
>>tzs+1e1
They can but they shouldn't.

Replace phone ownership with race or social-economic status and it should be obvious why profiling shouldn't be done.

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