Suppose that 12.7% of all births are preterm. (The gestation period of the pregnancy is less than 37 weeks.) Also 0.22% of all births resulted in a preterm baby who weighed 8 pounds, 13 ounces or more. What is the probability that a randomly selected baby weighs 8 pounds, 13 ounces or more, given that the baby is preterm? Is this unusual? 

Approach We want to know the probability that the baby weighs 8 pounds, 13 ounces or more, given that the baby was preterm. Because 0.22% of all babies weigh 8 pounds, 13 ounces or more and are preterm, P(weighs 8 lb, 13 oz or more and preterm) = 0.0022. Since 12.7% of all births are preterm, P(preterm) = 0.127. The phrase “given that” suggests we use the Conditional Probability Rule to compute the probability.