Bayes’ Theorem calculates the probability of one event happening given that another event has already happened. In terms of drug testing, this means the probability that a randomly selected person who has a positive test is in fact a drug user.
To perform the calculations, we need to know two things:
The accuracy of the drug test
The probability that a randomly selected person is a drug user, or the “base rate” at which drug use occurs in the population at large.