• We know that 20 percent of all messages were spam (the left circle) and 5 percent of all messages contained the word Viagra (the right circle).• We would like to quantify the degree of overlap between these two proportions. In other words, we hope to estimate the probability that both P(spam) and P(Viagra) occur, which can be written as P(spam ∩ Viagra).• Calculating P(spam ∩ Viagra) depends on the joint probability of the two events or how the probability of one event is related to the probability of the other.• If the two events are totally unrelated, they are called independent events.