1.1 - Defining Terms and History

Defining terms

Let's begin our study of epidemiology with some definitions.

The study of the distribution of disease and determinants of health-related states or events in specified human populations and the application of this study to the control of human health problems. (JM Last. Dictionary of Epidemiology. 2nd edition)
Clinical Epidemiology
The science of making predictions about individual patients by counting clinical events in similar patients, using strong scientific methods for studies of groups of patients to ensure that the predictions are accurate. (Fletcher, Fletcher, Wagner. Clinical Epidemiology. 1996)

What is the difference in these two views of epidemiology? In the clinical setting, epidemiologic methods are used to make a prediction about a health outcome for an individual based on scientific studies of groups of similar patients. Clinical epidemiology is integral to evidence-based medicine. Epidemiology itself is the study of disease in a population.with goals of determining the frequency and distribution of the disease as well as risk factors for the disease. Although epidemiology is defined with regard to human populations, epidemiologic principles can be extended to study other problems, such as colony collapse disorder in honeybees or improving herd health for a dairy.

Following links in the table below, explore selected events in the history of epidemiology and population health.

Selected History of Epidemiology and Population Health

  • 1849-54 - John Snow formed and tested hypothesis on the origin of cholera in London - one of the first studies in analytic epidemiology
  • 1910's - Flu pandemic
  • 1920 - Goldberger published a descriptive field study showing the dietary origin of pellagra
  • 1940's - Fluoride supplements added to public water supplies in randomized community trials
  • 1949 - Initiation of the Framingham study of risk factors for cardiovascular disease
  • 1950 - Epidemiological studies link cigarette smoking and lung cancer, demonstrating the power of case-control study design
  • 1954 - Field trial of the Salk polio vaccine - the largest formal human experiment
  • 1959 - Mantel and Haenszel develop a statistical procedure for stratified analysis of case-control studies
  • 1960 - MacMahon published first epidemiologic text with a systematic focus on study design
  • 1964 - US Surgeon General's Report on Smoking and Health establish criteria for evaluation of causality
  • 1970's - Large community based trials implemented, such as North Karelia and Stanford Three Communities; worldwide eradication of smallpox
  • 1980's - Chronic disease, injury and occupational epidemiology; HIV epidemic
  • 1990's - Behavioral risk factor epidemiology; prevention of adverse health outcomes through polices and regulations; national programs in breast and cervical cancer prevention; tobacco epidemiology; emerging infectious diseases; criticism of epidemiology for being inconsequential ('small' risk ratios); standardization of surveillance methods; Mad cow disease (BSE) in England and Europe; Variant Creutzfeld-Jacob disease; aging of USA; disaster epidemiology
  • 2000's - Genetic and molecular epidemiology; health disparities; racialism; HIPAA in the USA; West Nile Virus;
  • 2001 - 9/11
  • 2002 - bioterrorism; anthrax and smallpox threat and vaccinations
  • 2003 - SARS, quarantines and public health law; and world-wide epidemiology; BSE in Canada
  • 2004 - SARS recurrence; BSE in USA; flu epidemic
  • 2009--2009 H1N1 pandemic

General Dichotomies in Epidemiological Studies

When designing epidemiologic studies, choices must be made about the role of the investigator, the purpose of the study, the hypothesis regarding exposure and the unit of analysis. Here are some examples:

Role of investigator:

  • Observational – The investigator does not manipulate exposure of participants to risk factors. Most epidemiological studiesare observational OR
  • Experimental - According to the study design, the investigator manipulates the exposure of participants to some factor. Clinical trials and intervention studies are examples of such experiments. If the study participants themselves act to change their exposure to an influence, a natural experiment may occur. For example, a study of persone who have migrated from one environment to another could constitute a natural experiment.

Purpose of the study:

  • Descriptive - describes distribution of disease by time, place, person; used to generate hypotheses of disease causation or for health planning OR
  • Analytic - measures and tests the association between a hypothesized risk factor and a disease

Hypothesized Effect of Exposure

  • Harmful - exposure increases risk or presence of disease OR
  • Beneficial - exposure reduces risk or presence of disease

Unit of Analysis

  • Individual - the individual (e.g., person, animal) is the unit of analysis; potential to ignore the impact of the community or group effect on individual risk OR
  • Community - the community (e.g., county, hospital) is unit of analysis. There is potential for ecological fallacy in such studies. Lacking individual data, assuming that individuals perform similar to the average of the group may not be true.

Data for a typical epidemiologic study may be summarized in a table comparing numbers of cases (those with the disease or condition) to non-cases in terms of their exposure to a risk factor or beneficial agent. (Fig. 1)

Figure 1: 2 × 2 Epidemiologic Table


Case (Number)

Non-Cases (Number)
Total Exposure (Number)
Not Exposed