Visualizing False Positives In Broad Screening

AlejoHausner writes "To find one terrorist in 3000 people, using a screen that works 90% of the time, you'll end up detaining 300 people, one of whom might be your target. A BBC article asks for an effective way to communicate this clearly. 'Screening for HIV with 99.9% accuracy? Switch it around. Think also about screening the millions of non-HIV people and being wrong about one person in every 1,000.' The problem is important in any area where a less-than-perfect screen is used to detect a rare event in a population. As a recent NYTimes story notes, widespread screening for cancers (except for maybe colon cancer) does more harm than good. How can this counter-intuitive fact be communicated effectively to people unschooled in statistics?"

Comments

Re:Math ftl

[thisnamestoolong]

You, FTL. You screen 3,000 people, one of whom is a terrorist right, right? The test is wrong 10% of the time, right? That means that 10% of the time, when you screen a non-terrorist, you will get a false positive. There is also only a 10% chance that you will get a false negative when screening the terrorist, but you then have to pick him out from a field of 300 individuals, 299 of whom are false positives. Is this simple enough for your little brain or shall I spell it out further?

Re:Come On

[networkBoy]

that's because it's a well known fact* that ter'rists hate Nascar. It's also a well known fact** that ter'rists won't pay to get in to blow someone up. thus we can deduce with 100%*** accuracy that these guys could only be ter'rists in the breif period from when they left their car in the parking lot and walked to the gate. Furthermore, that they parked their car means they likely were not ter'rists because all ter'rists**** blow their cars up.
-nB

* And I'll put it on the internet if you ask me to quote it, and since it's on the net it must be true+
** again, if you call me out on this I'll just post it somewhere to quote it.
*** selective sample set (lies, damned lies, and statistics, right?)
**** just seemed good

+ I saw it on the internet somewhere, seriously.
-nB