glossary

In tests of validity, labeling cases or the presence of a characteristic incorrectly; that is, categorizing cases as noncases or failing to identify the characteristic when it is actually not present.

In tests of validity, labeling noncases or the absence of a characteristic incorrectly; that is, categorizing noncases as cases or identifying a characteristic when it is actually not present.

A statistical test of significance that can be used in place of a chi-square test when the expected number of subjects in some cells is less than five.

A diagram that shows progressive division of the original population into the groups from which inferences flow.

The concept that in order to prevent or abort a disease outbreak, a certain high proportion of individuals in a population must be resistant to an infectious disease agent.

High-risk groups

Groups in a community with an elevated risk of disease or other health-related event.

Historical cohort study

A cohort study done on a cohort defined in the past.

A person in whom infection may or may not occur.

A combination of the case-incident-control method and cohort method in one study.

A persistently high level of disease occurrence. See Endemic, Epidemic, Pandemic.

A statistically testable statement. See Null hypothesis.

The occurrence of new cases during a specified period of time.

A rate in which the numerator is person-years instead of the number of people because all subjects have not been followed for the same period of time.

See Cohort study

The period of time from exposure to the onset of symptoms. See Natural history of disease and Latency period.

A person who is capable of transmitting disease before being clinically ill (during the incubation period).

A relationship between two variables that holds when controlling or adjusting for other variables.

A characteristic or situation being considered for its relation (possibly causative) to a health outcome.

A dependent relationship between two variables, which appears to exist only because of the confounding influence of a third variable, and disappears when the third variable is controlled. See Secondary association.

Transfer of an agent from a reservoir to a susceptible host by suspended air particles or by animate intermediaries. See Vector and direct transmission.

Standard rates applied to populations being compared in order to calculate the expected number of events, and then compared with the observed number of events.

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.

The proportion of exposed persons who become infected.

Statistical techniques which allow one to analyze information and generalizes about a population on the basis of information obtained from a sample of a populations. See Descriptive statistics.

The degree to which two (or more) different observers classify consistently among themselves observations on a group of subjects.

Data that is measured in standard units where any given difference between two numerical values has the same meaning.

In chronic diseases, the period of time from exposure to the onset of symptoms. See Incubation period and Natural history of disease.

A method of including people in a cohort study for different durations during the overall study period.

When two variables are considered together, the first increases as the second increases (or decreases), so that, when plotted graphically, a straight line results.

A technique for controlling for variables that are actual or potential confounders.

An epidemic which has features of both common source and propagated epidemics, such as a common source outbreak followed by secondary person-to-person spread. See Common source outbreak and Propagated outbreak.

The means by which an infectious agent is conveyed from a reservoir to a host.

Rates used to express the incidence of disease or prevalence of disease. See Incidence rate and Prevalence rate.

The probability of death occurring within a specified time period multiplied by a constant.

Each subject is classifiable into only one category.

The description of what happens following the development of disease, including complications, cure, death, symptom change, remissions, and so forth. Systematic prospective study can quantify the probability of these events for a group of cases.

As the amount of the characteristic increases, the rate of the health state decreases.

A ratio expressing the number of deaths among children from birth 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.

The data from these variables label things; they do not measure or quantify amounts. Also called qualitative variables.

Statistical techniques that can be used on nominal or ordinal data.

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

The symmetrical clustering of values around a central location.

A statement expressing no relation or no difference between two (or more) variables.

The upper portion of a fraction. See Denominator and Rate.

A technique for estimating relative risk from case-control studies. See Relative odds ratio.

A variable having values that can be meaningfully ordered or ranked (for example, from less to more, from small to large).

A term meaning the same as epidemic; often used by public health officials because it is less alarming to the public than the term epidemic. See Epidemic.

An epidemic which spreads over several countries or continents and affects a large number of people. See epidemic.

The true value of a population’s health attribute.

Protection against a particular disease by acquiring antibodies either from one’s mother or by the receipt of injections of antitoxins or immune globulin. See Active immunity.

The proportion of infected persons who develop clinical disease.

The rate of cases of a disease or people with a health condition over a specified period of time. See Prevalence rate and Point prevalence.

Sometimes used in the denominator of incidence rates. It is the accumulated period of time each person is in a study. Person-years is sometimes used in the denominator instead of the number of people. See Incidence density rate.

In an experimental study a nontreatment of pseudo-treatment which is believed to be effective by the subject.

The prevalence of a disease of health condition at a particular point-in-time (i.e., the number or rate of individuals with a specific health outcome at the time period when the count is taken). See Prevalence rate and Period prevalence.

See Cross-sectional method.

When a group of people is exposed to a common noxious influence for a brief period of time, and everyone who becomes ill develops disease at the end of one incubation period. This is a classification of a common source outbreak. See Common source outbreak.

The whole group of individuals, objects, or measurements having some common observable characteristic. See Sample.

Those individuals capable of developing a specified health state; they become the denominator for calculating the rate of the health state.

A measure of the benefit derived by modifying a risk factor.

The path by which an agent enters to infect a susceptible host.

The place where an agent leaves its reservoir or host.

As the amount of the characteristic increases, the rate of the health state increases.

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

This exists if the findings have important implications within the conceptual context of the study.

The degree to which a measure forecasts the outcome of interest. See Validity.

The number or rate of cases that exist (prevail) at a specified time.

Rates that express in a given population the proportion of persons who have a particular disease or attribute at a specified point in time.

The prevalence for a risk factor for one group divided by that for another group.

An estimate of the frequency or likelihood of the occurrence of an event.

A disease outbreak which does not have a common source, but instead spreads gradually from person to person. See Common source outbreak.

The proportion of deaths attributable to different causes in a specified population over a period of time. 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.

See Prospective study and Cohort study.

See Cohort study.

The systematic collection, analysis, interpretation, and dissemination of health data on an ongoing basis.

The probability of obtaining a difference between sample estimates as large as the observed difference if the null hypothesis is true.

Variables that name or classify but do not indicate measurement. Also called nominal variables.

Variables that indicate measurement.

Research designs in which the investigator does not have as much control as in an experimental design.

Assigning people to treatment and control groups in an unbiased manner so as to produce groups similar in characteristics not controlled by other methods. Sometimes called random assignment of subjects to groups, or random allocation.

A clinical trial in which subjects have been randomly assigned to the various groups in the study.

A selection of some members of a population such that each member is independently chosen and has a known non-zero probability of being selected.

In a set of numbers or observations, the range is the largest observation minus the smallest.

A numerical statement of the frequency of an event obtained by dividing the number of individuals experiencing the event (the numerator) by the total number capable of experiencing the event (the denominator or the population at risk) and multiplying by a constant such as 100 or 1,000.

Data that have a true zero point that represents total absence of the variable.

See Association.

Percentages of cases that were exposed to a presumed antecedent divided by the percentage of controls that were similarly exposed. It is sometimes used in case-control studies as an estimate of relative risk.

A ratio obtained by dividing the incidence rate of one group by the incidence of another group. If the rates are equal, the resulting relative risk is one.

The degree to which observations are repeatedly classified the same.

See Reliability.

The habitat in which an infectious agent normally lives, grows, and multiplies.

Looking back. A retrospective study is a case-control study.

The probability or likelihood of a health-related event or outcome.

A characteristic, behavior, or experience that increases the probability of (causes) disease or other health-related event or condition. See High risk group.

A group of subjects chosen for study to represent a larger population.Sample size. The number of subjects chosen for study to represent a larger population.

The differences between findings for all possible samples from the same or equivalent populations. It is possible to draw many different samples from any one population, and each one would provide somewhat different findings.

Association produced by confounding variables. See Indirect association.

Refers to a trend over time.

Factors that affect the composition of the populations studied so as to confuse comparisons between groups, that is, produce artifactual findings.

Bias that is introduced into a study when study participants (subjects) are not representative of the population to which inferences are to be made. See Selection.

The result of differences between those who die and those who live; those who survive may have characteristics related to maintaining life that confound retrospective studies of the health outcome causing mortality.

See sensitivity.

In tests of validity, the percentage of all true cases identified correctly. See specificity.

See Attributable risk.

See specificity.

In tests of validity, the percentage of all true noncases identified correctly. See Sensitivity.

An irregular pattern of disease occurrence, with occasional cases occurring at irregular intervals. See endemic, Hyperendemic, epidemic, pandemic.

When applied to associations, false relationships produced by methodological errors or confounding variables.

The ratio of observed events to events expected if standard rates are applied to the study populations. It is usually the figure used in indirect adjustments.

The square root of the variance divided by the sample size. Describes how widely values are dispersed around the mean.

As arbitrary distribution of a characteristic (for example, age) used as a common standard for two groups when comparing their rates.

See adjusted rate.

Methods of determining the likelihood that estimates of population parameters are different solely because of sampling variability.

A way to present and summarize numerical data in an organized fashion.

A table that is complete except for the data.

The group a researcher wishes to study. Also known as a study population. See Random sample.

In tests of validity, labeling noncases or the absence of characteristics correctly.

In tests of validity, labeling cases or characteristics correctly.

Rejection of a null hypothesis that is actually true. See Alpha.

Failure to reject a null hypothesis that is actually false. See Beta.

Characteristics that in nature generally modify many health events and need to be considered when comparing groups.

An equal weight to each component of the average.

The correctness of labeling; the ability of a criterion or tool to measure what it claims to measure, or the correctness of participants’ reports.

A number that expresses the spread of individual data points around the mean for a sample.

Refers to an animate intermediary in disease transmission. Most vectors are arthropods such as mosquitoes, fleas, or ticks.

Objects such as food, water, biologic products (e.g., blood), and fomites (inanimate objects) that may indirectly transmit an infectious agent from a reservoir to a host.

Refers to the proportion of persons with clinical disease who become severely ill or die.

Different weights to each component of the average.

Years of potential life lost (YPLL)

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.

Infectious diseases that are transmissible under normal conditions from animals to humans.

A statistic computed to test the statistical difference between two numbers.


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