A beautiful illustration of this point is available in Chapter 1 of the book, Epidemiology for the Uninitiated. The authors provide the following example, a study, based on a survey of hang gliding accidents, recommended that flying should be banned between 11 am and 3 pm, because this was the time when 73% of the accidents occurred.

Conclusions from studies based upon descriptive data face an common epidemiologic problem. Namely, they only provide information on the numerator (the number of injuries) and little information on the denominator (the population at risk). In the example above, it is not correct to assume crashes are a problem from 11am to 3pm without first understanding the times and frequency in which all pilots (including those who do not crash) operate hang gliders. The number of hang glider accident cases was not related to the appropriate "at risk" population. Analytic epidemiologic studies focus on identifying causal factors for injuries from the population at risk. The populatin at risk includes persons who are injured and persons who are not injured.