You are out for a drive, when a policeman waves you over. He looks at your dashboard, and declares "I see that you have a full tank of gas. I'm giving you a ticket for speeding!" Sounds ridiculous, doesn't it? Yet, lots of otherwise sensible people make a similar error when they equate the increase in the percent of children with an ASD diagnosis (prevalence) with an increase the rate at which new cases of ASD are occurring (incidence). You have both a gas gauge and a speedometer on your dashboard, but you cannot equate one with the other; the same is true for prevalence and incidence of ASD.
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