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?"

Re:Math ftl

[Anonymous Coward]

Great! Thank you for identifying yourself as one of those "unschooled in statistics" people the summary mentioned. Now we just need to experiment with different ways to get you to understand this simple concept.

Re:Simple

[fuzzyfuzzyfungus]

The problem is motivating them to learn statistics.

I recommend accusing them of terrorism and sending them to a black site, where they'll be kept in a blank white cell with nothing but a statistics textbook. Don't release them until they can demonstrate that they are overwhelmingly likely to have been a false positive.

That'll learn 'em.

Re:Rare events.

[Anonymous Coward]

you insensitive clod - All I said was I was a Taoist and they arrested me because of my accent.

Re:Math ftl

[interkin3tic]

The test accuracy is measured compared to the population tested. In fact, a test that consistently says "no cancer" in all cases is 99% accurate when run on the general population.

Wow, thanks! I was going to have this mole checked out at the doctor, you really just saved me a lot of time! I mean, I didn't understand your magic numbers, but if it means I don't have cancer, I'm for it!

Broad Screening...

[CopaceticOpus]

I think they prefer the term "mammogram".

Re:Speech Recognition

[Thanshin]

That's easy, just tell them that the screenings work about as well as speech recognition. It's 95% accurate and everyone knows how much it sucks.

What R you toking about, Is peach recognitions the best since sly St.Bread?

Re:Come On

[somersault]

guess what they were NOT bothered at all

Yep, no hassle at all - in fact most people didn't even get closer than the blast radius from a decent sized stick of dynamite!

Re:DNA and fingerprints

[clone53421]

Barring twins and clones

Hey, I resent that!

Start with the basics

[RationalRoot]

A nurse recently told me a test was never wrong, it was 99% accurate. I asked how many people she had used the test on this week, she said about 50 a day. Without any knowledge of the population % of positives, and making the gross assumption that it was 99% false positive and 99% false negative, that would lead one to believe that she seeing incorrect results about 2-3 times a week. This had simply never occurred to her. Never mind the population statistic, nor the possible difference between false positive and false negative, but she understood 99% accurate to be - "never wrong". It never even occurred to her that if she was testing hundreds of people a week that some results would be just plain wrong. I didn't ever bother trying to explain the effect of population statistics.