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Mapping Rates

The rate of something, or the relative risk of something happening, may be very different from the number of events of this type that have happened, especially in their geographic distribution. For example, if you tell me that 10,000 Toyota Camry automobiles were stolen last year, I would ask, “well, what is the relative risk of a Camry being stolen?” Just knowing the number does not help me decide whether to buy a Camry based on whether or not it will be stolen. If there are 50,000 Camrys on the road, then the relative risk of a Camry being stolen is 10,000/50,000 or 0.20 (a percentage can be made by multiplying this result by 100; rates are often standardized by multiplying them by a constant, as in the crime rate per 100,000 population, or the divorce rate per 1000 married couples, and so on). On the other hand, if you also tell me that there are 100 Porche Boxters stolen in the same year, I will ask the same question-if there are only 250 Boxters on the road (at 60K per Boxter this could well be the case), you would say, “Oh, the Camry is much more likely to be stolen than the Boxter,” but you would be wrong because the relative risk for Boxters is 100/250 = 0.4. So, in fact, the relative risk of a Boxter being stolen is twice that of a Camry being stolen. The use of rates is a simple but powerful tool to gain understanding about the world around us.