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Next tool is the observation of the development of cancers in people. Work on the location of cancers and where they spread was mainly carried out in the nineteenth and early twentieth centuries and this process of careful description of what happens to patients, allied to very detailed descriptions of the behaviour of tumours, can still contribute useful knowledge. There remains a role for the doctor or group of doctors who keep meticulous records of the findings with large numbers of patients and the outcome for each and every one of them. This kind of clinical science is important but probably has now contributed most of what it can to our knowledge of cancer.

The third kind of method which helps us understand cancer is the study of the development of cancers in communities and of the links between the cancers seen in communities and other features of life in those communities, This is the science of epidemiology and it has, in many ways, been the most revealing of all of the sciences in the study of cancer in chit century (although many scientists practising in the laboratory might not share this view). In fact, epidemiology and experimental science and clinical observation are all complementary and, together, can tell us much about the different causes of cancer.

The study of the development of cancers in populations is best illustrated by the link between lung cancer and smoking. This topic merits a chapter in its own right, which will follow later, but the essence of the observations allows us to illustrate how epidemiology can work. As smoking increased in communities in Western Europe and North America so did lung cancer. Within those communities, it seemed to be mainly people who smoked who got lung cancer. Different groups within the society who had different exposure to cigarette smoking had different rates of lung cancer and when particular groups, for instance doctors, reduced their smoking, lo and behold, they got less lung cancer. This is a simple statement of a very complicated story and the net result has been to reveal the single most important known cause of human cancer, cigarette smoke. All of these observations were made by studying the patterns of cancer within communities, although they went extended by experiments in which the important elements of cigarette smoke, particularly ear, were mixed with cells and were shown to be capable of causing cancerous changes. The link between cigarette smoking and lung cancer remains the strongest clearest and most important link in our entire knowledge of cancer, and the one that presents the greatest potential for winning the war against cancer, at least on that front. In other areas, the links between cancer and lifestyle are much less clear, but they may turn out to be even more important; they certainly require very careful study.

In this chapter we are going to explain how a researcher can work on these problems, how certain of the observations he makes may be very compelling and also what the pitfalls and uncertainties may be. This will provide the basis for discussing each of the important potential causes of cancer, one by one, in ensuing chapters. The values and the limitations of epidemiology are an extremely complex topic and are frequently only well understood by people who are deeply immersed in the subject professionally. Misunderstandings about the strengths and weaknesses of this science have generated considerable confusion among the general public and also among healthcare professionals, and it is worth spending some time looking at how the suggestions of epidemiology are studied and evaluated, how they become more certain and how they may well remain unproven despite many years of careful work.