One measure of air travel safety is this: There are 117 fatalities per billion passenger flights. Express that measure as a probability. Is it unlikely for an air passenger to be a fatality? How does air travel compare to the car fatality rate of 40 fatalities per billion trips? Is this comparison fair? Consider an event to be “unlikely” if its probability is less than or equal to 0.05. (This is equivalent to the same criterion commonly used in inferential statistics, but the value of 0.05 is not absolutely rigid, and other values such as 0.01 are sometimes used instead.)