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Epidemiological research is an important tool for the improvement of preventive and curative healthcare. A valid study design, unbiased study conduct and proper interpretation of study results are essential, as these provide the foundation for evidence-based medicine. This book provides guidance on how to design, conduct and interpret human population-based medical research. It reflects the diversity in the field and will therefore discuss the methodology of etiological, diagnostic, prognostic and intervention research. This textbook is written for bachelor’s and master’s degree students receiving training in the biomedical sciences and for practitioners in the medical and allied professions. It will also serve as a reference manual for those with basic or advanced experience of epidemiology.

Commissioned by the European Epidemiology Federation.

Inhoudsopgave

Voorwerk

1. Epidemiology

‘Epidemiology’ is a term that not many people are familiar with: most will trip over the word when they try to pronounce it for the first time. Many professionals in healthcare and health research have only a vague notion of what it means. This is certainly not the case with the concept of an ‘epidemic’, which in most people conjures up a picture of a sudden outbreak of an infectious disease such as influenza or an intestinal disorder. It will come as no surprise to learn that the familiar term ‘epidemic’ and the relatively unfamiliar term ‘epidemiology’ are closely related. Associating these two concepts can be instructive, but it can also be dangerous, as an overly narrow interpretation of ‘epidemic’ can easily cause misunderstanding as to what epidemiology really is and what present-day epidemiologists study.
L. M. Bouter, G. A. Zielhuis, M. P. A. Zeegers

2. Frequency

As the previous chapter made clear, epidemiology is concerned with the study of disease frequency in human populations, usually in relation to one or more determinants. In formal terms this means estimating the epidemiological function.
L. M. Bouter, G. A. Zielhuis, M. P. A. Zeegers

3. Association

As we have seen in previous chapters, the epidemiological function represents the question being addressed by an epidemiological study as a mathematical formula. The epidemiological function links the disease outcome (frequency) to one or more determinants.
L. M. Bouter, G. A. Zielhuis, M. P. A. Zeegers

4. Study design

This chapter explains how analytical epidemiological studies should be designed in order to find the link between a determinant and a health outcome. A variety of approaches can be used: for example, investigating the influence of factors on the development of a particular condition (an etiological study); investigating the influence of factors on the course of a disease (a prognostic study); investigating the extent to which medical tests give correct predictions (a diagnostic study) or the efficacy of preventive or therapeutic interventions (an intervention study).
L. M. Bouter, G. A. Zielhuis, M. P. A. Zeegers

5. Validity and reliability

You will undoubtedly have noticed that characterizing the epidemiological function is pivotal in epidemiological research.
L. M. Bouter, G. A. Zielhuis, M. P. A. Zeegers

6. Etiology and causality

What does it mean if we say that A is the cause of B? The simplest idea of causality is that we mean that B always follows A, but there are two reasons why this cannot be correct. First, a causal relationship need not be perfect: smoking is a cause of lung cancer, but not all smokers get lung cancer. Secondly, even a perfect association is not enough to be regarded as causality: every time the cock crows the sun rises, but this does not of course mean that the cock’s crowing causes the sun to rise.
L. M. Bouter, G. A. Zielhuis, M. P. A. Zeegers

7. Genetic epidemiology

When looking for determinants of a disease we divide them, broadly speaking, into three categories: biology, behaviour and environment (see chap.​ 3). Genetic epidemiology is specifically concerned with the first category, and with the complex interactions between genetic factors and environmental or behavioural factors. It is thus closely related to molecular epidemiology, which is concerned with the role of biological or molecular markers of exposure and susceptibility, including DNA. Once the human genome had been mapped, interest in genetic determinants of disease – and hence in genetic epidemiological research – increased enormously. This is not the reason for devoting a separate chapter to this subspecialty in a general textbook of epidemiology, however: after all, this book does not include chapters devoted to the epidemiology of cardiovascular disease, psychiatric epidemiology, nutritional epidemiology or pharmacoepidemiology, to name but a few of many subspecialities. The reason for having a separate chapter on genetic epidemiology lies in the existence of specific biological mechanisms that apply to the transfer of genetic traits from one generation to the next. These make it possible to tackle questions regarding the genetic determinants of disease differently and more efficiently than other types of determinants. Consequently, genetic epidemiology uses some substantially different study designs and analytical methods in addition to the general epidemiological methods.
L. M. Bouter, G. A. Zielhuis, M. P. A. Zeegers

8. Outbreak epidemiology

An outbreak occurs when the number of new cases of a disease observed in a particular situation and a particular geographical area over a relatively short period is larger than would be expected based on the descriptive epidemiology of that disease (chap.​ 1). An increase in the incidence of a disease in a large population and/or area is referred to as an epidemic. We therefore need to have a good idea of the number of cases that can be expected in a particular period, place and population, and what variability is normal. Proper surveillance is very important, then, if we want to be able to recognize and investigate true outbreaks.
L. M. Bouter, G. A. Zielhuis, M. P. A. Zeegers

9. Diagnostic and Prognostic Research

This chapter looks at epidemiological research for diagnostic and prognostic purposes. Diagnosis and prognosis are two of the basic elements of medicine and paramedicine. The Greek word ‘diagnosis’ means ‘distinction’, and the aim of the diagnostic process is indeed to distinguish between health and disease in individuals, between different diseases that appear similar, or between different stages of a particular condition in patients. The aim of prognosis is to predict the course or outcome of a disease process after the patient has been diagnosed: it is concerned with such questions as the likelihood of cure, of permanent disability or of death within a particular time frame. Both the diagnostic process and prognosis are descriptive in nature and relate to individuals. This is the main difference between this chapter and the other chapters: here we are not interested in explaining things in terms of cause and effect. On that topic we would refer to the chapters on observational and experimental research. This chapter is about describing the likelihood that a particular disease or health outcome is present (diagnosis) or will occur (prognosis) in an individual. Taking it to extremes, if we were to find that the length of the big toe shows whether a person will recover completely from a cerebral infarction, the length of the big toe would have major prognostic value. Whether the length of the big toe is also a causal factor is irrelevant.
L. M. Bouter, G. A. Zielhuis, M. P. A. Zeegers

10. Intervention

It will not always be certain whether the aim of a preventive, therapeutic or palliative intervention has actually been achieved. And if it has, was that because of or in spite of the intervention? Again, these are really questions of cause and effect (see chap.​ 6). This chapter discusses research into the outcomes of medical interventions that aim to have a beneficial effect on the occurrence or course of a disease (for example vaccinations, health information, rules on diet and lifestyle, reducing pollution, water fluoridation, medication, surgery and psychotherapy).
L. M. Bouter, G. A. Zielhuis, M. P. A. Zeegers

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