In the first part of her series Mona Okasha discusses why epidemiology is worth bothering about

Have you ever played the word association game? You know the one, where I say a word and you say whatever comes into your head first. I say "Friday" and you say "pub," or I say "holiday" and you say "sun." OK, try this one then. I say "epidemiology" and you say "boring," "irrelevant," "rubbish." Even if none of those was the first word that came to mind I expect you may agree with one or all of them. Many medics feel the same, possibly because the relevance of epidemiology is rarely highlighted in medical curriculums or perhaps because the excitement of doctoring beats the boredom of statistics.

It needn't be like that though. Epidemiology can be useful, interesting, and fun. It could even shape your future career. In this article I will give a brief overview of what epidemiology is and illustrate how epidemiology is relevant to you as future doctors. I will also contest the challenge that epidemiology is useless and serves only to scare the public without adding much to our knowledge of health and disease. The second article will cover types of epidemiological study and where each of those types fits in the "hierarchy of evidence." After that there will be a step by step guide to understanding and interpreting epidemiological findings. Each of these articles will challenge the idea that epidemiology is boring. The final article will discuss whether epidemiology has passed its heyday or whether there still is a future for the discipline.

What is epidemiology?

There are numerous definitions of epidemiology, and I am not going to reproduce them here-go and check out your lecture notes if you want to know. Instead I am going to give you my impression of what epidemiology is. You may not agree with me, but that's fine. It is just my opinion.

Real life

Most of epidemiology is simply a matter of observing and describing real life, hence the term observational epidemiology. Some experiments also fall under the umbrella term of epidemiology - randomised controlled trials (RCTs) - although those are just a special case really. We still observe the participants, although the setting for such a study is less natural and more controlled.

Mainly, we just watch, make some observations, do some statistics (number calculations), and think about what our results could mean. Sometimes, it is necessary to do more than watch - we may need to ask folk to answer questions for us. Take a recent study on the association between orgasms and mortality. A cohort of men were asked about the frequency of experiencing orgasms (though not directly observed, this still counts as observational epidemiology!). These men were then kept track of over 10 ten years to see which of them died and which did not. Read on to find out whether sex is good for you.

Is epidemiology relevant to me as a doctor?

You are likely to be studying medicine because you want to become a doctor. So why would you want to know anything about epidemiology? You are going to be a hands on clinician, not a stuffy academic, aren't you? As a clinician you will be faced with difficult choices on a daily basis - can epidemiology really help?

Yes, is the answer. You will want to know which treatment works best for your patient - and the answer will probably have come from the results of an RCT. RCTs suggest that on a population level streptokinase may be indicated for patients who have just had a heart attack, but cannot guarantee the survival of the patient in front of you today. "Not much use, then," you may suggest. I don't agree. Although epidemiology does not have the answer for your particular patient, it provides you with the scientific basis on which to form your clinical decision.

Or you may want to contest the decision of your hospital chief executive that a new treatment ought to become the treatment of choice for a particular condition - and the way to do it is a systematic review of the epidemiological literature. Or a woman with a strong family history of breast cancer asks you how she can reduce her risk of developing the disease. Epidemiology can offer ways to determine modifiable risk factors that involve less radical action than removing both her breasts.

Most research articles are epidemiological in design

You may try to resist, but you cannot escape. Epidemiology is everywhere. Take as an example the journals that you are most likely to read as a doctor: the BMJ and the Lancet. Most research articles are epidemiological in design, recent articles ranging from a family tree study of people with high cholesterol levels to an RCT comparing medical with surgical treatment of children with severe kidney disease.

It is not only in the medical press that epidemiology is so widespread. Reports of epidemiological studies appear frequently in the lay media, and unfortunately these are not necessarily always reported well. Journalists often hone in on results of analyses which were not the main aim of the study, and may present these out of context. There have also been occasions when results have been wrongly reported as a result of journalists not understanding basic epidemiological concepts. For example, a study of hormone replacement therapy use and the risk of breast cancer found an annual increased risk of 2.3% among users compared with non-users. This was reported in the Times (5 October 1997) under the title, "HRT link to breast cancer proved" as a 2.3 times higher risk among users compared with non-users. Although subsequently corrected (9 October 1997), the front page article is clearly wrong and frightened the public unnecessarily. Many of you are likely to become general practitioners and will be faced in your daily practice by patients who have heard media reports of epidemiological findings. Your patients will expect informed and educated replies to their queries, which require you to have read and understood the epidemiological article, not the media's interpretation of it.

In the era before the introduction of more sanitary living conditions and the advent of antibiotics, epidemiology was primarily the study of infectious diseases. The focus of epidemiology is now shared between infectious and chronic diseases, although the balance in the developed world is towards the former and in developing countries remains tipped towards the latter. For those of you who choose to organise your elective in a developing country, or to work for a non-governmental organisation (NGO) overseas once you have qualified, the importance of epidemiology will be very evident. How do you deal with an outbreak of haemorrhagic fever or cope with a cholera epidemic in a refugee camp? These are real life situations that you may be faced with, in which the principles of epidemiology will be of utmost relevance.

Can epidemiology be exciting?

I hope that by now you are beginning to realise the varied uses that epidemiology may have for you as a clinician. Throughout, I have assumed some degree of hostility towards epidemiology, based on my own experience of the attitudes of clinical doctors. For some of you at least, I may have been preaching to the converted. Perhaps you have already discovered the excitement of epidemiology and realised the uses that it has in determining public health programmes and policies. For some of you, epidemiology will become your career. The importance of public health cannot be overstated. It is through public health measures that the work you do can affect more than just the patient in front of you. It can affect the health of many thousands of people, not only in your health authority or health board, but also on an international scale.

Time, place, and person

Epidemiology can describe patterns of disease in terms of who gets what, where, and when. For example, we observe that lung cancer is most frequent among male smokers. What traditional epidemiology cannot and does not try to do is identify the components of tobacco smoke which are harmful, although that may change in the post-genome mapping era. But in terms of public health, what is important is to get smokers to quit - and better - to stop people starting to smoke. It matters less what constituent of tobacco smoke is bad for you, and more that we can reduce the risk of cancer and heart disease by reducing smoking levels.

More harm than good

A criticism of epidemiology is that all we do is create and promote media health scares. Today it's coffee that may cause depression, tomorrow it's computer terminals that may cause breast cancer. Epidemiology gets into the lay media easily - the results are easy to understand, pertinent to people's day to day lives, and it makes good journalism. With people churning out this stuff all the time (and they do - it is amazing what gets published in the more obscure medical journals), is it a surprise that we get criticised for scaremongering?

However, epidemiology has a large number of uses, from the determination of the safety and efficacy of drugs, the causation and spread of diseases, to the evaluation of patients' satisfaction with health services. Can we really allow irresponsible research to obscure the important research? I don't advocate epidemiological studies to determine the relationship between all conceivable exposures and diseases, but I do advocate serious epidemiology with a sensible biological or sociological backing.

Sex, death, and epidemiology

Having read this far, you probably want to know the answer - is sex good for you? In the study of middle aged men mentioned above, those who had frequent orgasms were at half the risk of dying over the study period compared with those who rarely had sex.1 Great news, isn't it? Sex is good for you!

But epidemiological results cannot be taken at face value. In the next few articles I will reflect on the idea that sex makes you live longer. I will explore how epidemiological studies can be of use in determining the relationships between exposures and outcomes and what tools we can employ to decide how true what we see may be.

Further reading
Taubes G. Epidemiology faces its limits. Science 1995;269:164-9. Beaglehole R, Bonita R, Kjellström T. Basic epidemiology. WHO: Geneva, 1993. Barker DJP, Cooper C, Rose G. Epidemiology in medical practice. 5th ed. Churchill Livingstone: New York, 1998.