CASE STUDY On February 29, 2012, a woman in Provo, Utah, gave birth on a third consecutive Leap Day, tying a record set in the 1960s. The Associated Press picked up the story, and it was run in newspapers around the country as an amazing feat. If birth dates are random and independent, a statistician can show that the chance that three children, selected at random, are all born on Leap Day is about 1 in 3 billion. The rarity of the event is what made the story newsworthy.

Just how amazing is this event? In this chapter, you will learn how to interpret probabilities like 1 in 3 billion. By the end of this chapter, you will be able to assess coincidences such as having three children born on Leap Day. Are these events as surprising as they seem?