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

I can offer up a nice book on that

[Shivetya]

http://www.amazon.com/Manga-Guide-Statistics-Shin-Takahashi/dp/1593271891

I hate math, always did. I was good at it but just could not stand it. As such I skipped out on about anything math related beyond algebra (college level). Didn't impede my programming ability at all.

Still there are times where I like to learn how stuff works and honestly this series of books, Manga Guide to ......, has given me a quick leg up on a few subjects I would never have gained from traditional text books.

Re:Granted

[Dunbal]

addendum - I shouldn't have hit submit yet sorry

      Where it says "Assuming again a 90% accuracy, you are still wrongly intimidating well over 3500 people." I should add "per group of 35,000". 10% of 420 million passengers per year is 42 million people per year being harangued for no reason at all.

Re:Simple

[DavidTC]

Well, that example sorta fails because there are no actual terrorists in it.

A more sane example might be to get a group of a few hundred people, in, say, a auditorium, give them an envelope with a sheet of instructions, and a 20-sided die.

The instructions tell them if they're a terrorist or not, and tells them to roll the die and, depending on what it lands on, go to a specific labeled area. I.e., everyone goes to an area, differently, depending on what they rolled. You need about 20 areas.

Terrorists, of which you have a few, have a 9/10 chance to be sent to a 'terrorist' area, and everyone else has a 1/10 chance.

Everyone rolls, and goes to wherever, and at the end you realize which area was 'prison' and who got caught, and you ask everyone in that area to reveal their status, and you also ask all the other terrorists in other groups to step forward.

Or you could do this with badges and colored stickers, or something. The important thing is that people who are identified as 'terrorists' don't actually know they are until they're pointed out, which is a powerful psychological effect. (Which also means they shouldn't know who is a terrorist, or they'll realize it when grouped together.)

Could be a fairly powerful demonstration when people realize that 90% of the 'terrorist' group are innocent, and, while that group did catch most actual terrorists, there are a few still roaming around.