Let's say the false positive rate for drug testing is 8% and the false negative rate is 13%. (My Googling finds 5-10% and 10-15% respectively.)
Also let's say 10% of the population is using illicit drugs. (My Googling finds for example, 10% of the entire population has used illicit drugs recently.)
So you do drug testing on 1000 people. There are 900 clean people and 100 drug users.
100 x 0.13 = 13 people will show clean who are actually drug users. 87 people who are users will test correctly.
900 x .08 = 72 innocent people will wrongly show false positive. 828 people will test clean who are indeed clean.
So does this mean if you test positive, there is only an 8% chance you are innocent? Most people think so but it's not true:
If you have a positive test, what are the odds of you being innocent? Here are the people who test positive:
87 drug users
+72 innocent people
159 people test positive for drugs. Of those 159 people, 55% are drug users, and 45% are innocent.
This means if you test positive, you are ALMOST 50/50 CHANCE INNOCENT.
This seems counter intuitive because you know the false positive rate of the individual test is only 8%. But because the majority of the population are not drug abusers, your odds of being in the larger non-user population greatly increase the chance you are one of the false positives over the test's 8%.
The 8% accuracy becomes more true if you limit those you test to people with known histories of drug abuse. But when you test the population at large, it becomes extremely unfair. Random drug testing of all employees is an atrocity. It is feel-good overkill that hurts a lot of innocent people. There are much more accurate and fair ways to root out the true rotten apple.
Feel free to run these numbers again plugging in whatever statistics you come up with.
To the OP, it doesn't matter. Go to the concert. You've got almost as high a chance as testing positive for no reason at all as for your contact high.
