Christine L. Case, Ed.D.
Biology Professor
Skyline College


Glossary of Epidemiology Terms



MEAN, ARITHMETIC. The measure of central location commonly called the average. It is calculated by adding together all the individual values in a group of measurements and dividing by the number of values in the group.

MEAN, GEOMETRIC. The mean or average of a set of data measured on a logarithmic scale.

MEASURE OF ASSOCIATION. A quantified relationship between exposure and disease; includes relative risk, rate ratio, odds ratio.

MEASURE OF CENTRAL LOCATION. A central value that best represents a distribution of data. Measures of central location include the mean, median, and mode. Also called the measure of central tendency.

MEASURE OF DISPERSION. A measure of the spread of a distribution out from its central value. Measures of dispersion used in epidemiology include the interquartile range, variance, and the standard deviation.

MEDIAN. The measure of central location which divides a set of data into two equal parts.

MEDICAL SURVEILLANCE. The monitoring of potentially exposed individuals to detect early symptoms of disease.

MICROORGANISM. A living organism too small to be seen with the naked eyel includes bacteria, fungi, protozoa, algae, and viruses.

MIDRANGE. The halfway point or midpoint in a set of observations. For most types of data, it is calculated as the sum of the smallest observation and the largest observation, divided by two. For age data, one is added to the numerator. The midrange is usually calculated as an intermediate step in determining other measures.

MODE. A measure of central location, the most frequently occurring value in a set of observations.

MORBIDITY. Any departure, subjective or objective, from a state of physiological or psychological well-being.

MORTALITY RATE. A measure of the frequency of occurrence of death in a defined population during a specified interval of time.

MORTALITY RATE, INFANT. A ratio expressing the number of deaths among children under one year of age reported during a given time period divided by the number of births reported during the same time period. The infant mortality rate is usually expressed per 1,000 live births.

MORTALITY RATE, NEONATAL. A ratio expressing the number of deaths among children from birth up to but not including 28 days of age divided by the number of live births reported during the same time period. The neonatal mortality rate is usually expressed per 1,000 live births.

MORTALITY RATE, POSTNEONATAL. A ratio expressing the number of deaths among children from 28 days up to but not including 1 year of age during a given time period divided by the number of lives births reported during the same time period. The postneonatal mortality rate is usually expressed per 1,000 live births.


NATURAL HISTORY OF DISEASE. The temporal course of disease from onset (inception) to resolution.

NATURALLY ACQUIRED ACTIVE IMMUNITY. Antibody production in response to an infectious disease.

NATURALY ACQUIRED PASSIVE IMMUNITY. The natural transfer of antibodies, for example, transplacental transfer.

NECESSARY CAUSE. A causal factor whose presence is required for the occurrence of the effect (of disease).

NOMINAL SCALE. Classification into unordered qualitative categories; e.g., race, religion, and country of birth as measurements of individual attributes are purely nominal scales, as there is no inherent order to their categories.

NONCOMMUNICABLE DISEASE . A disease that is not transmitted from one person to another.

NORMAL CURVE. A bell-shaped curve that results when a normal distribution is graphed.

NORMAL DISTRIBUTION. The symmetrical clustering of values around a central location. The properties of a normal distribution include the following: (1) It is a continuous, symmetrical distribution; both tails extend to infinity; (2) the arithmetic mean, mode, and median are identical; and, (3) its shape is completely determined by the mean and standard deviation.

NOSOCOMIAL INFECTION. An infection that develops during the course of a hospital stay and was not present at the time the patient was admitted.

NUMERATOR. The upper portion of a fraction.


OBSERVATIONAL STUDY. Epidemiological study in situations where nature is allowed to take its course. Changes or differences in one characteristic are studied in relation to changes or differences in others, without the intervention of the investigator.

ODDS RATIO. A measure of association which quantifies the relationship between an exposure and health outcome from a comparative study; also known as the cross-product ratio.

ORDINAL SCALE. Classification into ordered qualitative categories; e.g., social class (I, II, III, etc.), where the values have a distinct order, but their categories are qualitative in that there is no natural (numerical) distance between their positive values.

OUTBREAK. Synonymous with epidemic. Sometimes the preferred word, as it may escape sensationalism associated with the word epidemic. Alternatively, a localized as opposed to generalized epidemic.


PANDEMIC. An epidemic occurring over a very wide area (several countries or continents) and usually affecting a large proportion of the population.

PASSIVE IMMUNITY. Specific antibodies obtained during the life of an individual; not produced by the individual.

PATHOGENICITY. The ability of a microorganism to cause disease by overcoming the defenses of the host.

PCR. Polymerase chain reaction; a technique using DNA polymerase to make multiple copies of a DNA template in vitro.

PERCENTILE. The set of numbers from 0 to 100 that divide a distribution into 100 parts of equal area, or divide a set of ranked data into 100 class intervals with each interval containing 1/100 of the observations. A particular percentile, say the 5th percentile, is a cut point with 5 percent of the observations below it and the remaining 95% of the observations above it.

PERIOD PREVALENCE. The amount a particular disease present in a population over a period of time.

PERSON-TIME RATE. A measure of the incidence rate of an event, e.g., a disease or death, in a population at risk over an observed period to time, that directly incorporates time into the denominator.

PIE CHART. A circular chart in which the size of each ``slice'' is proportional to the frequency of each category of a variable.

POINT PREVALENCE. The amount of a particular disease present in a population at a single point in time.

POPULATION. The total number of inhabitants of a given area or country. In sampling, the population may refer to the units from which the sample is drawn, not necessarily the total population of people.

PREDICTIVE VALUE POSITIVE. A measure of the predictive value of a reported case or epidemic; the proportion of cases reported by a surveillance system or classified by a case definition which are true cases.

PREVALENCE. The number or proportion of cases or events or conditions in a given population.

PREVALENCE RATE. The proportion of persons in a population who have a particular disease or attribute at a specified point in time or over a specified period of time.

PROPAGATED OUTBREAK. An outbreak that does not have a common source, but instead spreads from person to person.

PROPORTION. A type of ratio in which the numerator is included in the denominator. The ratio of a part to the whole, expressed as a ``decimal fraction'' (e.g., 0.2), as a fraction (1/5), or, loosely, as a percentage (20%).

PROPORTIONATE MORTALITY. The proportion of deaths in a specified population over a period of time attributable to different causes. Each cause is expressed as a percentage of all deaths, and the sum of the causes must add to 100%. These proportions are not mortality rates, since the denominator is all deaths, not the population in which the deaths occurred.

PUBLIC HEALTH SURVEILLANCE. The systematic collection, analysis, interpretation, and dissemination of health data on an ongoing basis, to gain knowledge of the pattern of disease occurrence and potential in a community, in order to control and prevent disease in the community.


QUARANTINE. Period of detention of individuals to determine whether they have a communicable disease; restricted access to a place where a communicable disease exists.


RACE-SPECIFIC MORTALITY RATE. A mortality rate limited to a specified racial group. Both numerator and denominator are limited to the specified group.

RANDOM SAMPLE. A sample derived by selecting individuals such that each individual has the same probability of selection.

RANGE. In statistics, the difference between the largest and smallest values in a distribution. In common use, the span of values from smallest to largest.

RATE. An expression of the frequency with which an event occurs in a defined population.

RATE RATIO. A comparison of two groups in terms of incidence rates, person-time rates, or mortality rates.

RATIO. The value obtained by dividing one quantity by another.

RELATIVE RISK. A comparison of the risk of some health-related event such as disease or death in two groups.

REPRESENTATIVE SAMPLE. A sample whose characteristics correspond to those of the original population or reference population.

RESERVOIR. The habitat in which an infectious agent normally lives, grows and multiplies; reservoirs include human reservoirs, animals reservoirs, and environmental reservoirs.

RFLP. Restriction fragment length polymorphism; a fragment resulting from restriction-enzyme digestion of DNA.

RISK. The probability that an event will occur, e.g. that an individual will become ill or die within a stated period of time or age.

RISK FACTOR. An aspect of personal behavior or lifestyle, an environmental exposure, or an inborn or inherited characteristic that is associated with an increased occurrence of disease or other health-related event or condition.

RISK RATIO. A comparison of the risk of some health-related event such as disease or death in two groups.


SAMPLE. A selected subset of a population. A sample may be random or non-random and it may be representative or non-representative.

SCATTER DIAGRAM. A graph in which each dot represents paired values for two continuous variables, with the x-axis representing one variable and the y-axis representing the other; used to display the relationship between the two variables; also called a scattergram.

SEASONALITY. Change in physiological status or in disease occurrence that conforms to a regular seasonal pattern.

SECONDARY ATTACK RATE. A measure of the frequency of new cases of a disease among the contacts of known cases.

SECULAR TREND. Changes over a long period of time, generally years or decades.

SENSITIVITY. The ability of a system to detect epidemics and other changes in disease occurrence. The proportion of persons with disease who are correctly identified by a screening test or case definition as having disease.

SENTINEL SURVEILLANCE. A surveillance system in which a pre-arranged sample of reporting sources agrees to report all cases of one or more notifiable conditions.

SEX-SPECIFIC MORTALITY RATE. A mortality rate among either males or females.

SKEWED. A distribution that is asymmetrical.

SIGNS. Changes due to a disease that a person can observe and measure.

SPECIFICITY. The proportion of persons without disease who are correctly identified by a screening test or case definition as not having disease.

SPORADIC. A disease that occurs infrequently and irregularly.

SPOT MAP. A map that indicates the location of each case of a rare disease or outbreak by a place that is potentially relevant to the health event being investigated, such as where each case lived or worked.

STANDARD DEVIATION. The most widely used measure of dispersion of a frequency distribution, equal to the positive square root of the variance.

STANDARD ERROR (OF THE MEAN). The standard deviation of a theoretical distribution of sample means about the true population mean.

SUFFICIENT CAUSE. A causal factor or collection of factors whose presence is always followed by the occurrence of the effect (of disease).


SURVIVAL CURVE. A curve that starts at 100% of the study population and shows the percentage of the population still surviving at successive times for as long as information is available. May be applied not only to survival as such, but also to the persistence of freedom from a disease, or complication or some other endpoint.

SYMPTOMS. Changes in body function that are felt by a patient as a result of a disease.


TABLE. A set of data arranged in rows and columns.

TABLE SHELL. A table that is complete except for the data.

TRANSMISSION OF INFECTION. Any mode or mechanism by which an infectious agent is spread through the environment or to another person.

TRANSOVARIAN. Through the ovary; transmission from the maternal organism through the egg to individuals of the next generation.

TREND. A long-term movement or change in frequency, usually upwards or downwards.


UNIVERSAL PRECAUTIONS. Recommendations issued by CDC to minimize the risk of transmission of bloodborne pathogens, particularly HIV and HBV, by health care and public safety workers. Barrier precautions are to be used to prevent exposure to blood and certain body fluids of all patients.


VACCINE. A preparation of killed, inactivated, or attenutated microorganisms or toxoids to induce artificially acquired active immunity.

VALIDITY. The degree to which a measurement actually measures or detects what it is supposed to measure.

VARIABLE. Any characteristic or attribute that can be measured.

VARIANCE. A measure of the dispersion shown by a set of observations, defined by the sum of the squares of deviations from the mean, divided by the number of degrees of freedom in the set of observations.

VECTOR. An animate intermediary in the indirect transmission of an agent that carries the agent from a reservoir to a susceptible host.

VIRULENCE. The degree of pathogenicity of a microorganism.

VITAL STATISTICS. Systematically tabulated information about births, marriages, divorces, and deaths, based on registration of these vital events.


YEARS OF POTENTIAL LIFE LOST. A measure of the impact of premature mortality on a population, calculated as the sum of the differences between some predetermined minimum or desired life span and the age of death for individuals who died earlier than that predetermined age.


ZOONOSIS. An infectious disease that is transmissible under normal conditions from animals to humans.