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Epidemiology Glossary

Epidemiology ResourcesBias

A bias is a flaw in either the study design or data analysis that leads to an erroneous result.

Case fatality rate

Case fatality rate is a mathematical quantity that describes the severity of a disease Y. Usually applied to acute diseases, it is the proportion of persons diagnosed with a disease Y who actually die from disease Y during the period of observation:

CFR in population p  =    # dying from disease Y in population p
# with disease Y in population p

Thus, diseases with high CFRs can be considered to be more severe than those with low CFRs.

Comparison of rates

A rate of disease is a measure of incidence or occurrence of disease in a population. The incidence rate is computed as follows:

Incidence in population p  =    # who develop the disease Y in population p in a time t
# in population p

In order to measure the association between a factor X and a disease Y, one compares the rates of Y in those exposed to X with the rates of Y in those not exposed to X. The rate comparison can be relative, known as a Rate Ratio or Relative Risk(RR), or absolute, known as a Rate Difference (RD).

RR  =    Rate of Y among those exposed to X
Rate of Y among those not exposed to X
RD  =   (Rate of Y among those exposed to X)-(Rate of Y among those not exposed to X)
Conditional probability

This is the probability of an event given (or conditional upon) the occurrence of another event, e.g. the probability of disease Y given exposure to factor X.


Confounding is considered a "mixing of effects". When a factor X causes a disease Y, that relationship could be confounded by a factor C that is associated with both factor X and disease Y. C would be an alternative explanation for the relationship observed between X and Y.

Contingency table

A table (usually 2 rows and 2 columns) that is often used in epidemiology to show the relationship between disease and exposure. The table can be used to divide persons into the categories of diseased and exposed (a), diseased and not exposed (b), not diseased and exposed (c), and not diseased and not exposed (d):

Yes No
Disease Yes a b
No c d
Decision-making about risk

Risk can be thought of as a probability of developing disease given a set of circumstances. In epidemiology, we can calculate risks associated with various factors. We are often concerned about how well people know the risks associated with various factors (behaviors, agents, etc) and how clearly they judge the harm or benefit associated with a given outcome - as well as the extent to which they use that knowledge when making their health-related decisions.

Disease transmission

Disease transmission describes the process of spread of a disease agent through a population. Investigations of disease transmission answer the questions: Who got it? and How did it spread (either from a common source or from person A to person B)?

Epidemic investigation

Many diseases have a baseline (endemic) incidence rate in the population, i.e. at any point in time some people are acquiring a given disease in the population, e.g. influenza (flu). An epidemic or outbreak is a greater incidence of the disease in a population than would normally have been expected, i.e. greater than the endemic incidence. Epidemiologists are often called upon to conduct research (investigations) to determine the causes and possible control measures for outbreaks of disease.


Epidemiologists are bound by ethical guidelines that dictate justice and fairness in all aspects of their work. Epidemiologists must consider the fair and just treatment of their study subjects, as well as accuracy and honesty in statistical analysis, interpretation and dissemination of study data.


Exposure is the generic term used to describe the effective presence of any agent or factor that is thought to cause disease, e.g. toxic chemicals, dietary habits, activity levels, microorganisms.

Hypothesis testing

When epidemiologists conduct studies to evaluate disease patterns or relationships, they often uncover associations between exposures and disease. Hypothesis testing is the epidemiological process by which researchers ask the question: " Does the association that I have observed represent a causal relationship between the putative exposure and the disease?"

Morbidity, mortality

Morbidity is the term used to refer to illness and mortality refers to death. They are often used in describing types of disease rates, e.g. morbidity rates (rates of illness) and mortality rates (death rates).

Population controls

Controls are the names given to individuals in epidemiologic research who serve as the references against which cases are compared. For example, in a study of whether a high fat diet causes breast cancer, one may compare the fat intake of the cancer cases to the fat intake of controls (non-cases). If fatty diets cause breast cancer, one would expect that there would be proportionately more women with fatty diets among the cases than among the controls. Population controls are controls drawn from the general population, as contrasted with controls drawn from specific subgroups such as hospital controls or family controls.


When measuring health-related phenomena in an epidemiologic study, it is important to achieve the highest levels of precision and accuracy. Precision refers to the degree to which there is variation in a measurement. Accuracy refers to the degree to which the measurement is , on average, correct. If each time a phenomenon is measured, the value is the same, but all measurements are far away from the true value, there is high precision but low accuracy. If measurements vary widely, but their average is close to the true value, there is accuracy but not precision.

Prevalence and incidence

Prevalence is a mathematical quantity that describes the presence of a disease Y in a population. It is the proportion of persons in the population with the disease Y:

Prevalence in population p  =    # with disease Y in population p at a given time
# in population p

Incidence is a mathematical quantity that describes the occurrence of a disease Y in a population. It is the proportion of persons in the population who newly develop the disease Y within a given time period (t):

Incidence in population p  =    # who develop the disease Y in population p in a time t
# in population p

So prevalence is a measure of all cases of disease at a point of time, and incidence is the measure of new cases of disease in a time period.

Racial and ethnic health disparities

This phrase is used to describe differences in disease rates by race/ethnicity. There are very few diseases for which those differences result from biological factors, e.g. sickle cell anemia in African Americans. The differentials in other diseases can usually be traced to socioeconomic phenomena, e.g. access to health care, economic issues, and discrimination, to name a few.

Relative risk, attributable risk

These are some of the more common measures of association (MA). A MA is any mathematical measure that is used to quantify the association between two or more variables; thus, it is a measure of the extent to which variables X and Y are related.

Attributable Risks (AR) are a group of measures which describe the amount of disease risk that can be attributed to a given factor X. One of the most common AR measures is the Attributable Fraction, which is the proportion of disease Y in the population that can be attributed to factor X. It is computed as follows:

[(Incidence in population) - (Incidence in unexposed group)]/ (Incidence in population)

Science and policy

In epidemiology, we engage in scientific inquiry, using the scientific method to describe disease patterns, to determine disease causation, and to devise effective disease control methods. One common disease control method is the development or modification of health policy. Epidemiologic research can be useful in the development/modification and evaluation of local, state and national policies aimed at reducing population disease risk.


Screening is the process of early diagnosis of a disease, i.e. identification of disease or risk factors for the disease in its pre-symptomatic or pre-clinical stage. There are several well-known mass screening programs, e.g. cervical cancer screening (Pap smear). Screening is useful when diagnosis of a disease earlier than it would usually have been detected improves the outcome.

Study design, case-control study

Study design refers to the methodology that is used to investigate a particular health phenomenon, exposure-disease relationship, etc. There are several types of study designs. The case-control study is one of the classic epidemiologic designs. In this design, two groups of individuals (cases of disease and non-diseased controls) are assembled and information is collected retrospectively on their exposures.


Surveillance is a systematic method for continuous monitoring of diseases in a population, in order to be able to detect changes in disease patterns and then to control them.