Science teacher: One problem with drug screening

in a drug screen is a little more complicated–it tells you what percent of the students not inhaling are correctly identified at the same time that not inhaling. If a example is 99.9% specific and you’re not using the drugs tested, there is only a 1 in a thousand chance you wish be amiss. identified as a drug user.

So, you think, if a test is 99% sensitive and 99% specific, it’s a affected good test. And it is.
Then you say, well, hey, if you test positive, then I can be 99% sure you are using the drugs.
And you’d be 100% wrong.

Huh?

The accuracy of the test depends on what percent of the population is actually using drugs.

Let’s suppose you exact developed the Spiffy Spliff Test, a cheap, very extraordinary screen that is 100% easily affected and 99% specific.

You need to get FDA approval, so you are looking a benevolent group to donate their time and urine to your little nonprofit company incorporated solely to leaving out the yewt of America.

Let’s say an order of monks lives on an atoll in the middle of the Pacific. They use drug sniffing dogs to prevent in any degree marijuana future onto their isle. Still, one of the monks has end-stage cancer, so he has a recipe for medical marijuana, which he uses, um, religiously.

This is a excellent popular place for monks. 10,000 monks hot here, and one is a known marijuana user. Let’s say for the sake of argument that not one of the other monks has used marijuana in the past decade.

Now let’s test them. The test is 100% sensitive, so the one monk using reefer gets identified as of the like kind. So far, so good.

There are 9,999 more monks to be tested. If the test is 99% specific, then 1% of the remaining monks will be falsely identified as using mary jane.

1% of a tumid sum up leaves a great quantity of monks–about 100 of the remaining monks will test positive.

So now we have 101 positive tests, and only 1 monk has truly used grass. Despite a test that’s 100% sensitive and 99% specific, the vast majority (over 99%) of those that tested positive have never used ganja.

What suppose that the ordeal is 99.9% specified? Well, then about 10 monks will be falsely positive. For every true postive (the cancer-stricken monk), we have 10 monks on the verge of getting kicked out of the monestary for “wrong” results.

I know this is counterintuitive. Still, in order for the touchstone to be accurate, you need a fairly elevated proportion of the monks to be hanging out bhind the Wawa.

What if 20% of the monks are potheads? Let’session crunch the numbers again.

20% of 10,000 is 2000, so right off the bat we have a couple many express tests. 8000 monks are left. If the test is 99% specific, then 1% of these 8000 monks, or 80, will test falsely positive. In this case, only 80 out of the 2080 (or about 4%) tests will be false positive.

Same test, drastically different false positive rate.

Take fireside message? The predictive value of a unsalable article screening test, even a actually excellence one, depends on how many kids in the inhabitants are actually using drugs.

Until people can wrap their heads around the testing, piss belongs in a toilet, not a test tube.