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In the performance of analytical studies epidemiologists move from the demanding chores of collecting accurate information into the realms of designing studies that seek to answer important individual questions about the causes of cancer. In this area they will usually have an idea to test - a hypothesis about some possible causative factor. The focus shifts from whole nations or whole regions to a much more closely defined group of individuals. By collecting a great deal more information about a rather smaller number of people (but not so small that our conclusions might be based on pure chance), it is possible not only to demonstrate links between particular factors and particular cancers but also to look carefully to see if there are any possible alternative links which have to be considered or excluded by careful work. A number of methods of performing analytical epidemiology are recognized and are worth mentioning to give the general flavour of this sort of work: cohort studies, case-control studies and intervention or experimental studies.

Cohort Studies. A group of people (usually hundreds or even thousands) are identified who have either been exposed 10 a particular risk factor or who may become exposed to some risk during the study. They arc then followed up carefully and the development of the particular cancer or cancers under study is recorded and compared to that in a similar group of people who have not suffered any exposure to the relevant risk. Although this sounds like a simple operation, collecting together the information on an adequate number of people and following them up for a long enough period is difficult, laborious and expensive. These studies may look at groups of people who are known to have been exposed to a risk already - workers exposed to a chemical for instance. On the other hand, they may take a group of people in a job and then look at their exposure to a risk as it develops. Further follow-up then checks for cancer incidence. This second kind of study - called a prospective cohort study - can be particularly accurate.



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