I Want to Hurt Your Brain

A few days ago, two esteemed guests, Heidi and Lindsay, asked for some help regarding a question which was posted on this site many moons ago. I asked the good readership to help Heidi and Lindsay out, which you did, in flying colours.

Specifically, a reader by the handle of *Anon*, if that is his real name, answered the question and then posed another question in the comment box that I thought you might enjoy. Here goes:

There's another cute one concerning drug testing. If a drug test has a 0.1% error rate for positive results and you are one of a thousand employees tested in a workplace, what are the odds your positive test result is correct?

It sounds simple enough, doesn't it? Don't be fooled, there's more than meets the eye.

Any guesses?