Uses of epidemiology
Social Medicine Research Unit, Medical Research Council.
Accepted 7 September 2007
Until 
1900 death rates in middle age were high and worsening (Figure 1a), but about the turn of the century sanitary reform began to show results in this age group. Mortality rates for both men and women began to fall, and they continued to fall fairly sharply until the 1920s. Then something happened. Female mortality maintained its downward course; but the reduction of male mortality slackened and almost stopped. One result of this is that death rates for these men, which were 10% higher than for women a hundred years ago, and 33% higher after the First World War, are now 90% higher. What happened? As we now know, many strange things were happening, and are reflected, in the vital statistics of the inter-war years. The most important was the emergence from obscurity of three diseases, particularly affecting males, and very common in middle age: duodenal ulcer, cancer of the bronchus and ‘coronary thrombosis’. The first of these is mainly important as a cause of morbidity; the other two are now major causes of death, killing annually over 20 000 middle-aged men. Figure 1b shows the figures for 1929–53, and the contribution of these two diseases to the course of mortality: the trend among men is very different without them.
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Figure 1a and b illustrate one use of epidemiology—in historical study. But first let me explain that what I am speaking of is the study of health and disease of populations and groups, the epidemiology of which Farr, Snow and Goldberger are the masters. In contrast with clinical medicine, the unit of study in epidemiology is the group, not the individual: deaths, or any other event, are studied only if information can be obtained, or inferred, about the group in which the events occurred. The clinician deals with cases. The epidemiologist deals with cases in their population. He may start with a population and seek out the cases in it; or start with cases and refer them back to a population, or what can be taken to represent a population. But always the epidemiologist ends up with some estimate of (cases/population). In consequence he can sometimes ask questions that the clinician may also ask, and get better or different information in reply. Sometimes he can ask questions that cannot be asked in clinical work at all. He can, for example, calculate the rates of occurrence, or frequency, of phenomena in the population—such as the deaths, from all and from particular causes, per 1000 aged 55–64, a hundred years ago and now, to make possible the kind of comparison shown in Figure 1a and b.
In this article I am considering epidemiology as a procedure for finding things out, of asking questions, and of getting answers that raise further questions—that is, as a method—and I will have less time to consider the results, the information, obtained in reply. I shall confine myself to the non-infectious diseases, and try to illustrate them mostly from investigations carried out from the Social Medicine Research Unit, or with material worked up in that unit. Seven ‘uses’ of epidemiology are described—different ways of looking at epidemiological data, or applications of the method.
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Historical |
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 Top Historical Community diagnosisThe individual's chancesCompleting the clinical pictureIdentification of syndromesClues to causesConclusionSummaryAcknowledgementsReferences |
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Historical statements made in medicine are of two broad kinds. The first describes the decline of infections, for example, and of nutritional deficiencies, and the main trends are usually very obvious. The other raises problems about the possible increase of various disorders, which is quite another matter. The questions usually put (‘Have disk syndromes become commoner?’ for example) are bedevilled by uncertainty about diagnosis and nomenclature in the past, and the lack of quantitative estimates of frequency at any time: how many cases occurred annually per 1000 men, aged x, in the 1930s and in the early 1950s? In such problems as the frequency of psychoneuroses, historical questions, which are often asked, are hopeless of direct answer; but even in disorders like leukaemia, urinary cancer or cerebral tumour, sub-arachnoid haemorrhage, dissecting aneurysm and the collagen diseases, it is exceedingly difficult to estimate how much a recent apparent increase reflects a true increase of disease, and how much it is the product merely of better recognition and greater availability of diagnostic services, etc. Such questions are clearly important because the role of environmental factors in aetiology, and of recent social change which may be associated with the increase, arises. As a result of a great deal of work, the increase of duodenal ulcer, cancer of the bronchus and coronary heart disease must now be accepted as a working hypothesis and guide to environmental study.
History in the making
Epidemiology may further be defined as the study of health and disease of populations in relation to their environment and ways of living. In a society that is changing as rapidly as our own, epidemiology has an important duty to observe contemporary social movements for their impact on the health of the population, and to try to assess where we are making progress and where falling back—an activity in line with the classic descriptions of famine and pestilence, of the relations of health and disease to social dislocations, wars and crises. What are the public health implications of the 1000 extra motor vehicles a day?; the modern distribution of poverty so different from the 1930s?; the sophistication of foods?; the rising consumption of sugar, our astonishing taste for sweets?; the derationing of fats?; more smoking in women?; more married women going out to work?; less physical activity in work and more bodily sloth generally?; multiple chemical and physical exposures, know and potentially hazardous?; the prodigious increase of medical treatments?; the 11-plus examination?; still increasing urbanization and sub-urbanization?; the rapid creation of new towns?; smokeless zones (still with sulphur)?; the building of new power stations? and what can we learn from other indicators of community health: crime, for example—the ups and downs of juvenile delinquency, and the apparent increase of sex crimes and of crimes of violence during a period when so much other crime is decreasing?
Some of these questions are being studied, some cannot yet be framed in scientific terms; but parts, at least, of some could be better tackled than they are. And there are even more fundamental problems in our society; perhaps epidemiology with its concern for woods rather than trees, its special ability to isolate major characteristics for study, can simplify the issues and usefully raise some bold questions about these, too. Indices of health are available, and their quality is improving, although many more are needed, particularly in ‘mental health’.
Looking ahead
For many the main interest of history is the light it can throw on the future. Vital statistics is better placed than most disciplines to forecast—for example, the whole population of old people of the second half of the century are already born and are leading their lives under the conditions we know. Figure 1a can therefore be projected ahead, if only with wide margins of confidence. What seems to be keeping the male death rate even as moderately satisfactory as it is now is the balancing of those diseases, which are increasing (such as ‘coronary thrombosis’) by those which are declining (tuberculosis and other infections). If the infectious diseases begin to reach some minimum before the modern epidemics are brought under control, or if their decline is halted, and if the large group of conditions that are relatively static (cancer of the stomach, cerebrovascular disease, etc.) do not show improvements in the meantime, the overall middle-aged male death rate will actually begin to rise. One consequence of this would be that the population of old people in the future will consist more and more of solitary old women (whatever the increasing popularity of marriage during recent years). The current trend of mortality in middle-aged males is the most striking feature of Western vital statistics. Very interestingly—another kind of epidemiological comparison—the situation is better in Scandinavia than in the English-speaking world, as illustrated by figures like these:
Mortality per 1000 aged 55–64 from all causes. (Mean of rates for separate countries. Latest available year)
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Searching questions need to be asked in this kind of situation. A first ‘reconnaissance’ suggests that there is no simple answer—all these populations, for example, have high living standards and nutritional levels.
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Community diagnosis |
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TopHistoricalCommunity diagnosisThe individual's chancesCompleting the clinical pictureIdentification of syndromesClues to causesConclusionSummaryAcknowledgementsReferences |
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Epidemiology provides the facts about community health; it describes the nature and relative size of the problems to be dealt with, and ‘maps’ are produced of such scales as are required or possible. Results are sometimes surprising—at any rate in contrast with the type of problem of which there is general awareness and concern in the public health movement. Over 10% of sickness absence in male industrial workers in 1951 was ascribed to ‘bronchitis’ (16 million days). ‘Psychological’ disorders accounted for >13 million days; gastric and duodenal ailments for over 11 million, ‘rheumatism and arthritis’ for over 11 million.3
Usually, however, we are concerned with the distribution of phenomena, and not merely their totals. Such distributions are firstly in terms of age and sex (race or colour, where applicable), economic status and so on. Table 1 is an example of a social-economic distribution in relation to primagravidae in Aberdeen. It shows some interesting similarities, as in nutrition, and the remarkable differences that still remain between the social classes in ‘capital’ goods like housing and education (in these early days of the Welfare State). There is a wide range of reproductive performance in this relatively homogeneous town. Such demonstration of inequalities between groups is a standard function of epidemiology, and it can be put to many uses—for example, in the same field as Table 1 to identify ‘vulnerable groups’ meriting special attention by health services (Figure 2).
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The individual's chances |
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TopHistoricalCommunity diagnosisThe individual's chancesCompleting the clinical pictureIdentification of syndromesClues to causesConclusionSummaryAcknowledgementsReferences |
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The risks to the individual—or at any rate their order of magnitude—of suffering an accident as a schoolboy cyclist or an elderly pedestrian, of developing leukaemia for a radiologist, of producing malformation from rubella or breast cancer from chronic mastitis, can be estimated only if the experience of whole populations of individuals is known and the relevant averages can thus be calculated. Figure 3 uses the method of the life-table, an easy and rather neglected technique, to give a rough idea of the ‘risks’ the average male in England and Wales now runs during his middle age, and it complements the picture of Figure 1a and b. It is in the light of something like a one-in-eight chance of suffering from coronary heart disease, one-in-ten from peptic ulcer, that the use of such terms as ‘epidemic’ have their warrant. About 33% of men reaching 35 years now die before they reach 65 years, compared with just over 20% of women. This approach is likely to become increasingly useful as forward-looking ‘prospective’ studies are initiated, for example, to try to learn something about the differences made to middle-age mortality by different ways of living.
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Operational research
The study of community health services—how they are working, what needs they are serving and how well, what they ought to be doing—is a slowly developing branch of social medicine little speeded by the war-time successes in rather different fields. Table 2 gives a few examples of simple analyses, using epidemiological methods, and the kind of questions (rather than answers) that emerge.
Why has the introduction of the National Health Service, in which for the first time every child has, or can have, a general practitioner, made so little difference to the school health service (Table 2, A)? What are the appropriate roles of school doctor and GP today? How much ‘family medicine’ can the general practitioner do if the children are treated elsewhere?
Is there enough ‘serious’ medicine to maintain keen clinical interest in general practice; how is the work divided between ‘serious’ and other problems (Table 2, B)?
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Since most attendances at these large and representative industrial medical officers’ clinics (Table 2, C) seem to be for ‘industrial’ reasons, who, it may be asked, does the industrial medicine in the great majority of factories and other work places where there is little or no industrial health service? What are the different elements in industrial medicine (carried out on the shop floor as well as in the clinic), and what is their relative importance, so that priorities for early advance can be planned?
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The diabetes figures (Table 2, D) show that social classes I and II did much better with the introduction of insulin than classes IV and V (this is seen throughout ‘young’ diabetes). How are the benefits of anti-coagulants being distributed today or of the new cardiac surgery? Differences are, I fancy, more likely to be regional and local than related to ‘social class’. And tonsillectomy? Is Glover's fantastic tale20 still true today? Do children in Leeds, Leicester and Exeter still run three times the risk of losing their tonsils as do the children of Manchester, Bradford and Gloucester? (And do these differences affect the children who most need to have their tonsils removed?) In how many other examples of medical, obstetric or dental care would such community comparisons stimulate fresh clinical thinking?
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Housing policy, pursuing the figures in Table 1, evidently does not mean in the comparatively prosperous city of Aberdeen, little affected by bombing, that young people of any social class are finding it at all easy to start a home of their own (Figure 4). Half of all families in 1951 were sharing dwellings. These housing figures are an illustration of the value of trying to base ‘operational research’ about social services on populations: the idea of the human needs the services are and should be meeting at once becomes important. (Not that the assessment of ‘needs’ is at all easy: so often ‘demand’ is revealed? Created—by supply. However, in the health services—school, maternity and child welfare, appointed factory doctor—which were established to meet needs that certainly have since changed and may have lessened, a re-assessment of the present situation is urgent. My private notion is that the Central Health Services Council might be armed with a research secretariat; otherwise I see no prospect of having enough ‘operational research’ carried out.)
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Completing the clinical picture |
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TopHistoricalCommunity diagnosisThe individual's chancesCompleting the clinical pictureIdentification of syndromesClues to causesConclusionSummaryAcknowledgementsReferences |
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The most obvious example of this function is the contribution of the epidemiological method in determining the sex and age incidence of disease. The calculation of accurate age-specific rates showing, for instance, that cancer of the ovary, and possibly of the breast, reach peak frequency late in middle age is of course a help in understanding these conditions, as are the relations with parity. But it is possible to go further. Epidemiology, being by definition concerned with all ascertainable cases in a population, often produces different pictures of disease from those derived only from hospitals, for example. Thus, half or more of the deaths of men from coronary heart disease in middle age (56% here) seem to occur in the first few days of the first clinical attack of ‘coronary thrombosis’ (Figure 5). A quite incomplete picture of coronary heart disease must result if many of these cases are excluded. But often these deaths are ‘sudden’, known only to the general practitioner and the coroner's pathologist. Special efforts are therefore needed to discover them, and, dependent on the success of such efforts, so may any picture presented of prognosis in this disease, of survival, and of the results of new treatments be very considerably modified.
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The same is probably true at the other end of the spectrum: to get an idea of how much there is of mild ischaemic heart disease, minor and maybe atypical, reliance cannot be placed on the cases that happen to turn up in a particular practice or out-patient department, but an inclusive and extensive study is needed. (This principle is made use of in ‘screening’ surveys to detect early sub-clinical disease. Thus detected, as in diabetes, progression may be halted, and the surveys are thus a measure of control or prevention.) In brief, studies of the natural history of disease will be more complete and correctly proportioned if based on all the cases satisfying specified diagnostic criteria occurring in a defined population. Pneumoconiosis, byssinosis, rheumatoid arthritis and nutritional disorders such as anaemia come to mind in this context.23–25
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Identification of syndromes |
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TopHistoricalCommunity diagnosisThe individual's chancesCompleting the clinical pictureIdentification of syndromesClues to causesConclusionSummaryAcknowledgementsReferences |
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This use again relates directly to clinical medicine. Broad descriptive clinical and pathological categories often include very different elements. Their different statistical distribution, and their different behaviour among the population, may make it possible to distinguish such elements from each other, and thus help to identify characteristic syndromes. Consider the mortality from ‘peptic ulcer’ in 1921–23 (Figure 6). Clearly there were at least two conditions to be studied—conditions with possibly different causes. My own main interest in this field is in trying to disentangle coronary heart disease form coronary atheroma, by study of their different distributions in the population today, and their different histories in the past 40 years.26
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Table 3 illustrates again from cardiovascular disease. The common lumping together of coronary and cerebrovascular lesions as ‘atherosclerosis’ is not very strongly justified in clinical or pathological terms. Nor do the two conditions always behave similarly epidemiologically: the recent vital statistics are quite different; and this small experience among doctors (Table 3) is interesting. The natural history of conditions as group phenomena may thus help to define syndromes. The vast unknown field of chronic chest disease—middle-aged men with respiratory symptoms—today offers particular opportunities for this application of the epidemiological method.(The reverse is also true—that the epidemiological method may help to show or to confirm that apparently disparate phenomena are connected, by drawing attention to their related behaviour in the population—for example, malformation and rubella, rheumatic fever and streptococcal infection, zoster and chicken pox. However, I cannot think of any satisfactory illustrations from the non-infectious diseases.)
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Clues to causes |
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TopHistoricalCommunity diagnosisThe individual's chancesCompleting the clinical pictureIdentification of syndromesClues to causesConclusionSummaryAcknowledgementsReferences |
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The main function of epidemiology is to discover groups in the population with high rates of disease, and with low, so that causes of disease and of freedom from disease can be postulated. The most obvious and direct examples are the original observations on the nutritional deficiencies (scurvy, beriberi, pellagra and goitre); the geographical study of cancer (especially of the skin and liver); the industrial cancers (bladder, for instance); and industrial accidents (of coal-miners or railway workers). The biggest promise of this method lies in relating diseases to the ways of living of different groups, and by doing so to unravel ‘causes’ of disease about which it is possible to do something.
‘Ways of living’ can usually be described only in simple terms, and the kinds of causes of health and disease postulated in them tend therefore to be in rather simple terms, not of intimate biological mechanisms, but of social factors in the satisfaction of elementary human needs, of large-scale environmental features, of major aspects of behaviour. They are thus often ‘general’ rather than specific factors of health, causes of dis-ease, or diseases, rather than of a particular disease: as in the relations of water supply to bowel infections (not merely the cholera), living space and respiratory infections (as a class), income levels, nutrition and growth. We are only just beginning to identify such factors in ways of life, mass habits and social customs that may be related to many of the important problems of our own highly advanced society. For example, overnutrition (obesity; and ? atherosclerosis, thrombosis, dental caries, diabetes and toxaemia of pregnancy); physical inactivity (? ‘coronary thrombosis’ and ? how much else); tobacco and smoking (oral and lung cancer—a ‘specific’, rather, this—‘coronary thrombosis’; and ? how much else); atmospheric pollution (lung cancer; ? chronic chest disease of many kinds, and ? acute, and ? other, non-respiratory conditions); cultural factors (genital cancer in Jews and non-Jews); eating customs (? alimentary cancer); ‘social isolation’ (? schizophrenia, senile psychosis and suicide). And, of course, there is still an overabundant heritage of 19th century (and earlier) poverty and crowding, insanitation and bad habits that contribute over-fully to modern ill health.
Figure 7 illustrates from some recent studies, and includes a wide range of data, from the first turning of the ground to highly advanced observations. I have included (b)28 the famous analysis from the ‘Metropolitan Life’ on the dangers of ‘overweight’; an elaboration of Doll and Bradford Hill's classic data on cigarette smoking and cancer of the lung (e);10,29 some figures (unique so far as I know) from France on consanguineous marriages in relation to stillbirth and early neonatal mortality (g)27 and example (h)14 of the recent pioneering by the General Register Officer in psychosis, the darkest area of all; figures from Revans on the size of industrial units in relation to the frequency of accidents (d),30 which raise large questions of morale and the human environment, of group functioning as such, rather than the properties of the individuals who aggregate the group (compare immunity, and endemicity of infections). The distributions shown (a)22,31 on physical activity (they are different for gastric ulcer, incidentally, and for diabetes in younger persons, and do not show with other diseases), and on the disadvantages of town dwellers in respect of respiratory disease (c),1,2 merely set a stage for further inquiry; the natural experiment, or ‘experiment of opportunity’ provided by miners and clerks (f)8 provides, at least, contrast groups in which it may be profitable to seek factors that are significant in modern infant mortality.
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The great advantage of this kind of approach to prevention is that it may be applicable in the early stages of our knowledge of diseases, to disrupt the pattern of causation before the intimate nature of diseases is understood. Sufficient facts may be established for this by epidemiological methods alone, or in combination with others. The opportunity may thus offer to deal with one ‘cause’, or with various combinations of causes. Moreover, the possibility of two types of control may be opened up—environmental (as in the fluoridation of water) and personal (through alteration of diet and hygiene).
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Conclusion |
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TopHistoricalCommunity diagnosisThe individual's chancesCompleting the clinical pictureIdentification of syndromesClues to causesConclusionSummaryAcknowledgementsReferences |
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Epidemiology is today the Cinderella of the medical sciences. The proposition might, however, be advanced that public health needs more epidemiology, and so does medicine as a whole, and, it may be said, society at large. Public health needs more epidemiology—this is the most obvious intellectual basis for its further advance. Epidemiology, moreover, as a tried instrument of research—with its modern developments in sampling and surveys, small-number statistics, the follow-up of cohorts, international comparisons, field experiment and family study; and with its extensions to problems of genetics as well as environment, to physiological norms as well as disease, the psychological as well as the physical, morbidity as well as mortality—epidemiology now offers the possibility of a new era of collaboration between public health workers and clinical medicine. Such a collaboration could be on equal terms, each making their particular contribution to the joint solving of problems. There is abundant evidence today that clinicians would very much welcome such a development.
Medicine as a whole needs more epidemiology, for without it cardinal areas have to be excluded from the consideration of human health and sickness. Epidemiology, moreover, is rich with suggestions for clinical and laboratory study, and it offers many possibilities for testing hypotheses emerging from these. One of the most urgent social needs of the day is to identify rules of health living that might do for us what Snow and others did for the Victorians, and help to reduce the burden of illness in middle and old age which is so characteristic a feature of our society. There is no indication whatever that the experimental sciences alone will be able to produce the necessary guidance. Collaboration between clinician, laboratory scientist and epidemiologist might be more successful. The possibilities are at present unlimited, if often neglected.
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Summary |
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TopHistoricalCommunity diagnosisThe individual's chancesCompleting the clinical pictureIdentification of syndromesClues to causesConclusionSummaryAcknowledgementsReferences |
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We may summarize what has been said in terms of some of the relations between epidemiology, the epidemiological method and clinical medicine.
Epidemiology studies populations, and all cases that can be defined in them. It is concerned not only with those whose troubles immediately present to particular clinical attention but with the sub-clinical, the undiagnosed, the cases treated elsewhere. It thus helps to complete the clinical picture and nature history of disease.
Epidemiology supplements the clinical picture: by asking questions that cannot be asked in clinical study—about the health of the community and of sections of it, present and past; by setting clinical problems in community perspective, describing their behaviour as group, not individual, phenomena, indicating their dimensions and distributions, and how much, and where, action is needed; by revealing problems and indicating where among the population these might best be studied.
Finally, epidemiology by identifying harmful ways of living, and by pointing the road to healthier ways, helps to abolish the clinical picture. This is its chief function and the one in greatest need of development today.
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Acknowledgements |
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TopHistoricalCommunity diagnosisThe individual's chancesCompleting the clinical pictureIdentification of syndromesClues to causesConclusionSummaryAcknowledgementsReferences |
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I am grateful to my coworkers outside and in the unit for much help; and in particular wish to thank Mr C Daly and Mrs VP Hall for help with the figures.
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Notes |
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Read in opening a discussion at the Section of Preventive Medicine and Infectious Diseases at the Annual Meeting of the British Medical Association, Glasgow, 1954, and since expanded. First published BMJ 1955;2:395–401. Reprinted with permission. 
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References |
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TopHistoricalCommunity diagnosisThe individual's chancesCompleting the clinical pictureIdentification of syndromesClues to causesConclusionSummaryAcknowledgementsReferences |
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