As others have said, possibly to make very disparate events comparable across a population, like the risk of skydiving vs. a shark attack. Unfortunately, this r

As others have said, possibly to make very disparate events comparable across a population, like the risk of skydiving vs. a shark attack. Unfortunately, this results in an almost useless measure for any particular individual when the probability of being in a risk group is small. Here is a breakdown of the risk for a random person in the US dying from skydiving in 2012 [0]: (1 death / 100,000 jumps) x (3 million jumps / 350,000 jumpers) x (350,000 jumpers / 313 million people) The last two terms give us the prior P(jump) ~= 0.01 and the first is the conditional probability P(death | jump) ~= 0.0001. The non-skydiver's risk factor is, of course, zero while the skydiver's risk is about 100x greater than that of the average person (who doesn't actually exist). As you can see, the population measure isn't very informative for either group. [0] http://adventure.howstuffworks.com/skydiving8.htm http://adventure.howstuffworks.com/skydiving8.htm