1.1 - Defining Terms

Major Definitions for the Study of Epidemiology Section

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 between these two views of epidemiology?

In the clinical setting, epidemiologic methods are used to predict 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 to determine the frequency and distribution of the disease as well as risk factors for the disease. Although epidemiology is defined concerning 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 farm.

General Dichotomies in Epidemiological Studies Section

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 the exposure of participants to risk factors. Most epidemiological studies are observational
  • 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 persons who have migrated from one environment to another could constitute a natural experiment.

Purpose of the study:

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

Hypothesized Effect of Exposure:

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

Unit of Analysis:

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

Data for a typical epidemiologic study may be summarized in a table comparing the numbers of cases (those with the disease or condition) to non-cases in terms of their exposure to a risk factor or beneficial agent. (2x2 Epidemiologic Table)

2x2 Epidemiologic Table
  Case Non-Case Total
Exposed A B Texposed
No Exposed C D Tnon-exposed
Total Tcases Tnon-cases