In the course of studying epidemiology, the epidemiologists have developed two main branches of their science, one essentially retrospective and the other prospective. Retrospective epidemiologists follow Pott's method, but at a much higher level of thoroughness and sophistication. Having identified a group especially high in a specific cancer or in cancers generally, they probe the lives both of the victims and of those who have escaped the disease. They analyze medical records and personal histories, and the personnel and health records kept by employers. Survivors are questioned on every conceivable detail of their working and living habits, and the investigators search through death certificates to identify earlier cases

In one typical retrospective project - conducted in 1981 - a team of researchers led by Dr.Brian MacMahon of the Harvard School Of Public Health studied 369 victims of pancreatic cancer and 644 subjects who were free of that cancer. The epidemiologists asked all the subjects about their use of tobacco, alcohol, tea and coffee, in an attempt to  discover whether any of these factors increase the risk of developing the disease. One important connection emerged: pancreas cancer patients were more likely to be coffee drinkers than were the controls. Neither alcohol nor tea increased the risk; cigarette smoking increased it only slightly. What was more, the more coffee a person drank, the greater the risk. Those who drank one or two cups a day were twice twice as likely to develop pancreas cancer as those who drank no coffee at all. Those who drank five or more cups tripled their risk. Obviously, the effects of such a study can be vital to cancer prevention. In the United States, more than half of the population over the age of 10 drinks coffee every day, and Dr.MacMahon estimated that more than half of all pancreas cancer could be linked to coffee drinking. 

Unlike Dr.Macmahon - who worked backward from existing records - prospective epidemiologists follow large numbers of people into the future. These people are not necessarily ill when the project starts. The epidemiologists simply records their medical histories and their life styles in great detail, then waits to see what happens to them. The waiting is expensive; hundreds or even thousands of technicians may keep close track of the subjects for decades. But the findings - based upon identical - exhaustively detailed observation of many people over a long period, are especially valuable.

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