1 Defining epidemiology
Epidemiology has been defined as ‘the study of how often diseases occur in different groups of people and why’ (Coggon et al., 2003, p. 1). It is concerned not only with the distribution of disease within a particular group or population, but also with the causes of disease. Epidemiologists are in many ways disease detectives (Bailey et al., 2005), tracking down disease to discover why it occurs as well as when and where it occurs. Although the term ‘epidemiology’ literally means the ‘science of epidemics’, it actually has a much broader remit and explains why and how diseases occur and spread, what causes them and how they can be contained. The types of events that are of interest to epidemiology include morbidity (disease), disability, mortality (death), recovery and the use of health services. There is also a whole area known as ‘psychiatric epidemiology’ where the distribution of, and risk factors for, mental illness are studied.
Epidemiology focuses on populations or communities. These can be very large, such as a nation or continent, or groups within larger populations. So a population such as that of the UK might be broken down into the four nations, or specified health districts. But epidemiology is concerned not just with geographical groupings: often it focuses on such diverse groups as different occupations or residents in nursing homes. The key feature is that it investigates a ‘population at risk’: ‘The population at risk is the group of people, healthy or sick, who would be counted as cases if they had the disease being studied’ (Coggon et al., 2003, p. 1). It is therefore important to ensure that the population under study is capable of having the disease or condition.
Thinking point: Why might an epidemiologist study pregnant women only?
A reason for studying pregnant women might be to investigate morning sickness or gestational diabetes or the use of antenatal facilities. Studies of prostate cancer, on the other hand, would exclude women.
Epidemiology is the scientific foundation for public health in that it assists in identifying the health problems in particular communities, can assess the relevance of prevention and evaluate the effectiveness of preventive interventions (Tannahill, 1994, p. 97). Public health practitioners rely, to a large extent, on epidemiological data to provide valuable information about the health of their population. It is therefore important to understand the principles behind epidemiological methods and techniques.
Methodologically, epidemiology adheres to the principles of the ‘scientific method’. This is the way of knowing that is considered to be acceptable within biomedicine. The research method embedded in this paradigm falls under the umbrella of quantitative research methods; that is, it emphasises the collection of objective quantifiable data which can then be subjected to statistical procedures. Before exploring some of the methods and procedures used in epidemiology, the origins of modern epidemiology are briefly examined.