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.
Intervention or Experimental Studies. These studies represent the most difficult but in many ways the most informative of all forms of analytic epidemiology.
Here, two very similar groups of people are identified, often by computer-generated methods, randomly allocating people into one or other of the groups. With the consent of the people involved, one half are asked to undertake a change in their lifestyle. Perhaps a particular dietary clement may be changed for that group alone, on the theory that that dietary element may be related to cancer. The people are then studied for a long period of rime to see if there is any difference in the medical outcomes between the two groups. Since the groups of people were essentially identical at the beginning, any differences that emerge arc very likely to be linked directly to the change that was introduced for one group alone. This it precise science which can give accurate answers, but it is fraught with many human problems. The first of these is ethical. If there is a strong and genuine suspicion that the intervention may be helpful, and that only one group will benefit from it, many physicians would feel it morally wrong to perform such studies. Equally, if, as is essential in all medical research, the people involved are fully informed of the experiment being undertaken, many may feel unwilling to join. This means that accruing numbers of people into these studies may prove difficult. It is also extremely difficult to choose the right point in time at which to undertake an intervention study. If the risk factor being studied is very likely to be associated with cancer, then the ethical constraints will make at too difficult to perform the study, since no researcher will want just one group of people to benefit from an intervention study which leaves the other group exposed to a highly probable risk. If, on the other hand, the information . about the risk factor is very preliminary and perhaps weak, then there may not be sufficient reason to deploy the resources required for an intervention study in order to evaluate it fully. In a few circumstances the right point has been identified and there are currently major debates about whether intervention studies to test 9 number of other important theories are appropriate.