Heston

You are called Heston. You need to make a bar of chocolate. The process to make this bar of chocolate uses three machines, one after the other. All three need to work for the chocolate bar to be made. Each machine has a success rate of 90%. What is the probability that the whole process is a success and the chocolate bar gets made?

Answer = 73%. (0.9 x 0.9 x 0.9 = 0.73)

People often guess a higher success rate than that. This is an example of the confirmation heuristic, whereby people overestimate the probability of conjunctive events. A 'heuristic' just means a rule of thumb, we use them all the time and mostly that's okay, but sometimes they lead us to make systematic mistakes just like the one above.