How That 'Extra .9%' Could Ward Off a Zombie Apocalypse

netbuzz writes "The questioner on Quora asks: 'When is the difference between 99% accuracy and 99.9% accuracy very important?' And the most popular answer provided cites an example familiar to all of you: service level agreements. However, the most entertaining reply comes from a computer science and mathematics student at the University of Texas, Alex Suchman. Here's his answer: 'When it can stop a Zombie Apocalypse.'"

Comments

Re:That's not the question

[deimtee]

It's not a virus, it's a parasitic protozoan that is common in cats.
It's called toxoplasmosis gondii and it makes men violent and women horny.
Rats also get it, and it makes them attracted to the smell of cat piss.

Re:Statistics 101

[suutar]

The key factor is that the trait being tested is rare; only one in 500 people has it. In this case, the false positives can still be (substantially) more frequent than true positives.

Say you test 50,000 people. 100 have it, 49,900 don't. Of the 100 who have it, there will be 99 correct 'yes' results and one incorrect 'no' result. And of the 49900 who don't have it, there will be 49401 correct 'no' results and 499 incorrect 'yes' results.

So total, we have 598 'yes' results. But 499 of those are false positives, which is 83.4444%; only 16.5555% of the folks who test positive are really positive.