Without pen and paper try to guess the answer to the following problem: "One in a thousand people has a prevalence for a particular heart disease. There is a test to detect this disease. The test is 100% accurate for people who have the disease and is 95% accurate for those who don't (this means that 5% of people who do not have the disease will be wrongly diagnosed as having it). 

If a randomly selected person tests positive what is the probability that the person actually has the disease?"

Answer: 2%. It should be positive for 50 people from 1000 but only for one of them the test will not lie (select the white text)


One more experiment for those who found the task interesting.