Box 1.1 Epidemiology is …
‘The science of epidemics’ (Concise Oxford Dictionary, 1964)
‘The science of the occurrence of illness’ (Miettinen, 1978)
‘The study of the distribution and determinants of disease in humans’ (MacMahon and Pugh, 1970)
‘The study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to control of health problems’ (Porta, 2008)
‘The study of the occurrence and distribution of health-related events, states and processes in specified populations, including the study of the determinants influencing such processes, and the application of this knowledge to control relevant health problems’ (Porta, 2014)
So what is epidemiology anyway? As shown in Box 1.1, the Concise Oxford Dictionary (1964) defined it accurately, but not very helpfully, as ‘the science of epidemics’. In 1970, MacMahon and Pugh came up with something a bit more concrete: ‘the study of the distribution and determinants of disease’. Their definition succinctly identifies the two core strands of traditional epidemiology: who is developing disease (and where and when), and why are they developing it? The next definition, from the 2008 edition of the Dictionary of Epidemiology (Porta, 2008), takes things two steps further by broadening the scope to include health in general, not just disease, as well as highlighting the essential role of epidemiology in translating research findings into health policy and medical practice to control disease. The most recent definition (Porta, 2014) elaborates further still but, in doing so, loses some of the elegance of the earlier versions.
Epidemiology, therefore, is about measuring disease or other aspects of health, identifying the causes of ill-health and intervening to improve health; but what do we mean by ‘health’? Back in 1948, the World Health Organization (WHO, 1948) defined it as ‘… a state of physical, mental and social well-being’. In practice, what we usually measure is physical health, and this focus is reflected in the content of most routine reports of health data and in many of the health measures that we will consider here.