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Water: Microbial

Thesaurus of Terms Used in Microbial Risk Assessment: 5.7 Epidemiology and Surveillance Terms

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absolute risk

An incidence rate, usually expressed per 1,000 individuals.  (NZ 2002)

aggregate surveillance

The surveillance of a disease or health event by collecting summary data on groups of cases, e.g., general practitioners taking part in surveillance schemes are asked to report the number of cases of specified diseases seen over a specified period of time.  (MERREA 2005)
RELATED TERMS: public health surveillance, surveillance

analytic epidemiology

The aspect of epidemiology concerned with the search for health-related causes and effects.  Uses comparison groups, which provide baseline data, to quantify the association between exposures and outcomes, and test hypotheses about causal relationships.  (CDC 2005)
RELATED TERMS: applied epidemiology

analytical epidemiologic study

An evaluation of the association between exposure to hazardous substances and disease by testing scientific hypotheses.  (EPA 2004)

applied epidemiology
  • The application or practice of epidemiology to address public health issues.  (CDC 2005)

The application and evaluation of epidemiologic discoveries and methods in public health and health care settings.  It includes applications of etiologic research, priority setting and evaluation of health programs, policies, and services.  It is epidemiologic practice aimed at protecting and /or improving the health of a defined population.  It usually involves identifying and investigating health problems, monitoring for changes in health status, and/or evaluating the outcomes of interventions.  It is general conducted in a time frame determined by the need to protect the health of an exposed population and an administrative context that results in public health action.  (MERREA 2005)
RELATED TERMS: analytic epidemiology

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attack rate
  • The proportion of an exposed population at risk who become infected or develop clinical illness during a defined period of time.  (FAO/WHO 2003b, ILSI 2000)
  • The proportion of a disease-free population that becomes ill during a stated or implied period of risk.  (NZ 2002)
burden of illness

The sum total incidence, severity, and duration of gastrointestinal disease are known as burden of these illnesses.  (Payment and Riley 2002)

carrier
  • The inert liquid or solid material in a pesticide product that serves as a delivery vehicle for the active ingredient.  Carriers do not have toxic properties of their own.  (EPA 2005b)
  • Any material or system that can facilitate the movement of a pollutant into the body or cells.  (EPA 2005b)
  • An individual who does not display the symptoms of a disease, but harbors the pathogen which causes it, or has the gene (or genes) for it, and can transmit the disease to others either through interacting with other individuals, or by passing the disease-causing gene (or genes) to offspring.  (CancerWEB 2005)
  • A person or animal without apparent disease who harbors a specific infectious agent and is capable of transmitting the agent to others.  The carrier state may occur in an individual with an infection that is inapparent throughout its course (known as asymptomatic carrier), or during the incubation period, convalescence, and postconvalescence of an individual with a clinically recognizable disease.  The carrier state may be of short or long duration (transient carrier or chronic carrier).  (CDC 2005)
  • A person or animal that harbors a specific infectious agent in the absence of discernible clinical disease and serves as a potential source of infection.  (MERREA 2005)
case
  • In epidemiology, a countable instance in the population or study group of a particular disease, health disorder, or condition under investigation.  Sometimes, an individual with the particular disease.  (CDC 2005)
  • In epidemiology, a person in the population or study group identified as having the particular disease, health disorder, or condition under investigation.  (MERREA 2005)
  • An individual who is ill following ingestion of food. Outbreak cases reported by CDC are determined to be contaminated on the basis of laboratory analysis and/or epidemiological evidence. Not all outbreak cases need be confirmed by laboratory analysis if there is sufficient epidemiological evidence linking them to the outbreak.  (USDA 2004)
case-control study
  • An epidemiologic study contrasting those with the disease of interest (cases) to those without the disease (controls).  The groups are then compared with respect to exposure history, to ascertain whether they differ in the proportion exposed to the chemical(s) under investigation.  (EPA 2003)
  • A study that compares exposures of people who have a disease or condition (cases) with people who do not have the disease or condition (controls).  Exposures that are more common among the cases may be considered as possible risk factors for the disease.  (ATSDR 2004)
  • A type of observational analytic study. Enrollment into the study is based on presence (“case”) or absence (“control”) of disease.  Characteristics such as previous exposure are then compared between cases and controls.  (CDC 2005)
  • A retrospective observational study designed to determine the relationship between a particular outcome of interest (e.g., disease or condition) and a potential cause (e.g., an intervention, risk factor, or exposure).  Investigators identify a group of patients with a specified outcome (cases) and a group of patients without the specified outcome (controls).  Investigators then compare the histories of the cases and the controls to determine the rate or level at which each group experienced a potential cause.  As such, this study design leads from outcome (disease or condition) to cause (intervention, risk factor, or exposure).  (NLM/NICHSR 2004)
  • An inquiry in which groups of individuals are selected in terms of whether they do (the cases) or do not (the controls) have the disease of which the etiology is to be studied, and the groups are then compared with respect to existing or past characteristics judged to be of possible relevance to the etiology of the disease.  (RAIS 2004, SRA 2004)
case definition
  • A set of standard criteria for deciding whether a person has a particular disease or health-related condition, by specifying clinical criteria and limitations on time, place, and person.  (CDC 2005)
  • The case definition is a standard set of criteria for deciding whether an individual should be classified as having the health condition of interest.  (Gregg 1996)

A set of diagnostic criteria that must be fulfilled in order to identify a person as a case of a particular disease. Case definition can be based on clinical, laboratory, or combined clinical and laboratory criteria, or a scoring system with points for each criterion that matches the features of the disease.  (MERREA 2005)
RELATED TERMS: syndrome

case-fatality ratio

A ratio of the number of deaths due to a disease to the number of cases of that disease in a specified period of time.  It expresses the frequency with which affected individuals die of the disease.  (SRA 2004)

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case study
  • A medical or epidemiologic evaluation of one person or a small group of people to gather information about specific health conditions and past exposures.  (EPA 2004, ATSDR 2004)
  • A brief fact sheet providing risk, cost, and performance information on alternative methods and other pollution prevention ideas, compliance initiatives, voluntary efforts, etc.  (EPA 2005b)

An uncontrolled (prospective or retrospective) observational study involving an intervention and outcome in a single patient.  (Also known as a single case report or anecdote.)  (NLM/NICHSR 2004)
RELATED TERMS: anecdotal data, anecdotal evidence

case-study epidemiologic study

SEE:case study

case-control epidemiologic study

SEE: case-control study

chronic study

A toxicity study designed to measure the (toxic) effects of chronic exposure to a chemical.  (EPA 2003)

clinical illness

Deviation from the normal healthy state, manifested as symptomatic disease.  (ILSI 2000)

clinical trial

Research study conducted with patients, usually to evaluate a new treatment or drug.  Each trial is designed to answer scientific questions and to find better ways to treat individuals with a specific disease.  (CancerWEB 2005)
RELATED TERMS: experimental study

cluster
  • An aggregation of cases of a disease or other health-related condition, particularly cancer and birth defects, which are closely grouped in time and place.  The number of cases may or may not exceed the expected number; frequently the expected number is not known.  (CDC 2005)
  • An aggregation of relatively uncommon events or diseases in space and/or time in amounts that are believed or perceived to be greater than could be expected by chance.  (MERREA 2005)
cluster investigation

A review of an unusual number, real or perceived, of health events (for example, reports of cancer) grouped together in time and location.  Cluster investigations are designed to confirm case reports; determine whether they represent an unusual disease occurrence; and, if possible, explore possible causes and contributing environmental factors.  (ATSDR 2004)

cohort
  • A group of people within a population that can be aggregated because the variation in a characteristic of interest (e.g., exposure, age, education level) within the group is much less than the group-to-group variation across the population.  (EPA 2004)
  • A well-defined group of people who have had a common experience or exposure, who are then followed up for the incidence of new diseases or events, as in a cohort or prospective study.  A group of people born during a particular period or year is called a birth cohort.  (CDC 2005)
  • A fixed population in which membership is permanent (in contrast to a dynamic population).  Also defined as a group of persons who experience a certain event in a specified period of time (e.g., a birth cohort of babies born in 1990 in New Zealand).  (NZ 2002)
cohort epidemiologic study

SEE: cohort study

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cohort study
  • An epidemiologic study comparing those with an exposure of interest to those without the exposure.  These two cohorts are then followed over time to determine the differences in the rates of disease between the exposure subjects.  (EPA 2003)
  • A type of observational analytic study.  Enrollment into the study is based on exposure characteristics or membership in a group.  Disease, death, or other health-related outcomes are then ascertained and compared.  (CDC 2005)
  • An observational study in which outcomes in a group of patients that received an intervention are compared with outcomes in a similar group, i.e., the cohort, either contemporary or historical, of patients that did not receive the intervention.  In an adjusted- (or matched-) cohort study, investigators identify (or make statistical adjustments to provide) a cohort group that has characteristics (e.g., age, gender, disease severity) that are as similar as possible to the group that experienced the intervention.  (NLM/NICHSR 2004)
  • A follow-up or longitudinal study that assesses exposure status before assessing outcome.  (NZ 2002)

An epidemiologic study that observes subjects in differently exposed groups and compares the incidence of symptoms.  Although ordinarily prospective in nature, such a study is sometimes carried out retrospectively, using historical data.  (RAIS 2004)
RELATED TERMS:
prospective study, retrospective study

control

In a case-control study, comparison group of persons without disease.  (CDC 2005)
RELATED TERMS: case, case-control study

control group
  • A group used as the baseline for comparison in epidemiologic studies or laboratory studies.  This group is selected because it either lacks the disease of interest (case-control group) or lacks the exposure of concern (cohort study).  (EPA 2003)

The set of observations in an experiment or prospective study that do not receive the experimental treatment(s).  These observations serve (a) as a comparison point to evaluate the magnitude and significance of each experimental treatment, (b) as a reality check to compare the current observations with previous observation history, and (c) as a source of data for establishing the natural experimental error.  (NIST/SEMATECH 2005b)
RELATED TERMS: reference group

cross-sectional study

An epidemiological study design in which measurements of cause and effect are made at the same point in time.  (SRA 2004)

death rate
  • An estimate of the portion of a population that dies during a specified period.  (MERREA 2005)

An estimate of the proportion of the population that dies during a specified period, usually a year; the numerator is the number of people dying, the denominator is the number in the population, usually an estimate of the number at the midperiod.  (Stedman 2005)
RELATED TERMS: mortality

descriptive epidemiologic study

An evaluation of the amount and distribution of a disease in a specified population by person, place, and time.  (EPA 2004)

disease surveillance

SEE: public health surveillance, surveillance

endemic
  • Present or usually prevalent in a population or geographical area at all times, said of a disease or agent.  (CancerWEB 2005)
  • The constant presence of a disease or infectious agent within a given geographic area or population group; may also refer to the usual prevalence of a given disease within such area or group.  (CDC 2005)
  • Something found in a particular people or location, such as a disease that is always present in the population.  (CRCWQT 2002)
  • Denoting a temporal pattern of disease occurrence in a population in which the disease occurs with predictable regularity with only relatively minor fluctuations in its frequency over time.  (Stedman 2005)
epidemic
  • Occurring suddenly in numbers clearly in excess of normal expectancy, said especially of infectious diseases but applied also to any disease, injury or other health related event occurring in such outbreaks.  (CancerWEB 2005)
  • The occurrence of more cases of disease than expected in a given area or among a specific group of people over a particular period of time.  (CDC 2005)
  • Widespread outbreak of a disease, or a large number of cases of a disease in a single community or relatively small area.  Disease may spread from person to person, and/or by the exposure of many persons to a single source, such as a water supply.  (CRCWQT 2002)

The occurrence in a community or region of cases of an illness, specific health-related behavior, or other health-related events clearly in excess of normal expectancy; the word also is used to describe outbreaks of disease in animals or plants.  (Stedman 2005)
RELATED TERMS: outbreak; contrast with endemic, epizootic

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epidemiology
  • The study of the distribution and determinants of health-related states or events in specified populations.  (EPA 2003)
  • The study of disease patterns in human populations.  (EPA 2004)
  • Study of the distribution of disease, or other health-related states and events in human populations, as related to age, sex, occupation, ethnicity, and economic status in order to identify and alleviate health problems and promote better health.  (EPA 2005b)
  • The study of the distribution and determinants of disease or health status in a population; the study of the occurrence and causes of health effects in humans.  (ATSDR 2004)
  • The study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems.  (CDC 2005)
  • A branch of medicine which studies the patterns of diseases in populations, and their causes. The objective of epidemiology is to understand how and why diseases occur so that ways can be developed to prevent or reduce disease.  (CRCWQT 2002)
  • The study of the occurrence and causes of diseases or other health-related conditions, states, or events in specified populations.  One of the chief functions of this study is to identify populations at high risk for a given disease, so that the cause may be known and preventive measures implemented.  (FDA 2001)
  • The study of the occurrence of disease, or other health-related variables, in human populations.  (NZ 2002)
  • The study of the distribution and dynamics of diseases and injuries in human populations. Specifically, the investigation of the possible causes of a disease and its transmission.  (RAIS 2004)
epidemiology triad

The traditional model of infectious disease causation.  Includes three components: an external agent, a susceptible host, and an environment that brings the host and agent together, so that disease occurs.  (CDC 2005)
Also referred to as the epi-triad.

epizootic
  • Veterinary equivalent of an epidemic.  (CancerWeb 2005)
  • Denoting a temporal pattern of disease occurrence in an animal population in which the disease occurs with a frequency clearly in excess of the expected frequency in that population during a given time interval.  An outbreak (epidemic) of disease in an animal population.  (Stedman 2005)
experimental study

A study in which the investigator specifies the exposure category for each individual (clinical trial) or community (community trial), then follows the individuals or community to detect the effects of the exposure.  (CDC 2005)
RELATED TERMS:
clinical trial

follow-up study

SEE: cohort study

general population

The total of individuals inhabiting an area or making up a whole group.  (EPA 1997a)

health outcome data

In a public health assessment, community-specific health information such as morbidity and mortality data, birth statistics, medical records, tumor and disease registries, surveillance data, and previously conducted health studies that may be collected at the local, state, and national levels by governments, private health care organizations, and professional institutions and associations.  (EPA 2004)

health outcomes study

In a public health assessment, an investigation of exposed persons designed to assist in identifying exposure or effects on public health.  Health studies also define the health problems that require further inquiry by means of, for example, a health surveillance or epidemiologic study.  (EPA 2004)

herd immunity
  • An estimate of exposure, or dose level received anyone in a defined population that is greater than the 90th percentile of all individuals in that population, but less than the exposure at the highest percentile in that population. A high end risk descriptor is an estimate of the risk level for such individuals. Note that risk is based on a combination of exposure and susceptibility to the stressor.  (EPA 2005b)
  • Resistance of a group to a pathogen due to immunity of a large proportion of the group to that pathogen.  (NLM 2005)
  • The resistance of a group to invasion and spread of an infectious agent, based on the resistance to infection of a high proportion of individual members of the group. The resistance is a product of the number susceptible and the probability that those who are susceptible will come into contact with an infected person.  (CDC 2005)
  • The immunity of a group or community.  The resistance of a group to invasion and spread of an infectious agent, based on the resistance to infection of a high proportion of individual members of the group. The resistance is a product of the number susceptible and the probability that those who are susceptible will come into contact with an infected person.  (MERREA 2005)
high-risk community

A community located within the vicinity of numerous sites of facilities or other potential sources of environmental exposure/health hazards which may result in high levels of exposure to contaminants or pollutants.  (EPA 2005b)

incidence
  • The number of new cases of a disease that develop within a specified population over a specified period of time.  (EPA 2003)
  • The number of new cases of disease in a defined population over a specific time period.  (ATSDR 2004)
  • The rate of occurrence of new cases of a disease or condition in a population at risk during a given period of time, usually one year.  (NLM/NICHSR 2004)
  • The number of new cases of a disease in a population over a period of time.  (RAIS 2004, SRA 2004)

A measure of the magnitude of a disease, usually expressed as the number of new cases of a disease per 100,000 individuals in the U.S. population in a one-year period.  (USDA 2004)
RELATED TERMS: contrast with prevalence

incidence rate
  1. The ratio of new cases within a population to the total population at risk given a specified period of time.  (EPA 2003)
  2. A measure of the frequency with which an event, such as a new case of illness, occurs in a population over a period of time.  The denominator is the population at risk; the numerator is the number of new cases occurring during a given time period.  (CDC 2005)
index case
  1. A person who first draws attention to their family.  For example, if my eye doctor discovers I have glaucoma and subsequently other cases of glaucoma are found in my family, I am the index case.  Also called the propositus (if male) or proposita (if female).  (CancerWeb 2005)
  2. In human genetics, the patient or member of the family that brings a family under study.  (Stedman 2005)

Note: Index case can also refer to the initial case in a larger population and not just within a family.

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instrument error

A type of non-sampling error caused by the survey instrument (or questionnaire) itself, such as unclear wording, asking respondents for information they are unable to supply or the instrument being changed in some way during the course of the research.  (ESOMAR 2001)
RELATED TERMS: survey

longitudinal epidemiologic study

SEE: cohort study

matching
  1. The process of making a study group and a comparison group in an epidemiological study comparable with respect to extraneous or confounding factors such as age, sex, weight, etc.  (CancerWEB 2005)

In a retrospective study, a method for identifying a comparison group.  Matching pairs observational unit: each unit that has both trait-of-interest A and nuisance effects B,C,... with another unit that lacks trait-of-interest A, yet still shares B,C,... Low yielding lots (trait-of-interest is yield) are in this way compared to well yielding lots of the same product started at about the same time.  Matches in this way are more sensitive to key causal differences (for example, in the particular equipment set used) than would occur from taking “matches” from all available lots.  Matching is a way of implementing commonality studies.  Matching is a kind of blocking for retrospective studies.  (NIST/SEMATECH 2005b)
RELATED TERMS: case-control study, retrospective study

morbidity
  1. State of being ill or diseased.  Morbidity is the occurrence of a disease or condition that alters health and quality of life.  (ATSDR 2004)
  2. Any departure, subjective or objective, from a state of physiological or psychological well-being.  (CDC 2005)
  3. A departure from a state of physical or mental well-being, resulting from disease or injury. Frequently used only if the affected individual is aware of the condition.  Awareness itself connotes a degree of measurable impact.  Frequently, but not always, there is a further restriction that some action has been taken such as restriction of activity, loss of work, seeking of medical advice, etc.  (MERREA 2004, RAIS 2004, SRA 2004)
morbidity

The number of illnesses or cases of disease in a population in relation to the total population.  (NYS 1998)
RELATED TERMS: incidence

mortality
  1. Death.  Usually the cause (a specific disease, a condition, or an injury) is stated.  (ATSDR 2004)

Death; the death rate; ratio of number of deaths to a given population.  (RAIS 2004, SRA 2004)
RELATED TERMS: death rate

mortality rate
  1. A measure of the frequency of occurrence of death in a defined population during a specified interval of time.  (CDC 2005)

The number of deaths that occur in a given population during a given time interval; usually deaths per l03 or l05 people per year.  Can be age, sex, race, and cause specific.  (MERREA 2005, RAIS 2004, SRA 2004)
RELATED TERMS: death rate

notifiable disease
  1. Diseases, usually of an infectious nature, whose occurrence is required by law to be made known to a health officer or local government authority.  (CancerWEB 2005)
  2. A disease that, by statutory requirements, must be reported to the public health authority in the pertinent jurisdiction when the diagnosis is made.  A disease deemed of sufficient importance to the public health to require that its occurrence be reported health authorities.  The reporting to public health authorities of communicable diseases is, unfortunately, very incomplete.  The reasons for this include diagnostic inexactitude, the desire of patients and physicians to conceal the occurrence of conditions carrying a social stigma, and the indifference of physicians to the usefulness of information about such diseases as hepatitis, influenza, and measles.  Notifications provide the starting point for investigations into the failure of preventive measures, such immunizations, for tracing sources of infection, for finding common vehicles of infection, for describing the geographic clustering of infection, and for various other purposes, depending upon the particular disease.  (MERREA2005)
observational epidemiologic study

A study in which the investigators do not manipulate the use of, or deliver, an intervention (e.g., do not assign patients to treatment and control groups), but only observe patients who are (and sometimes patients who are not as a basis of comparison) exposed to the intervention, and interpret the outcomes.  These studies are more subject to selection bias than experimental studies such as randomized controlled trials.  (NLM/NICHSR 2004)
RELATED TERMS: observational study

observational study

Astudy in which the investigators do not manipulate the use of, or deliver, an intervention (e.g., do not assign patients to treatment and control groups), but only observe patients who are (and sometimes patients who are not as a basis of comparison) exposed to the intervention, and interpret the outcomes.  These studies are more subject to selection bias than experimental studies such as randomized controlled trials.  (NLM/NICHSR 2004)
RELATED TERMS: observational epidemiologic study

occurrence

In epidemiology, a general term describing the frequency of a disease or other attribute or event in a population without distinguishing between incidence and prevalence.  (MERREA 2005)
Occurrence is also used in the context of pathogen occurrence, which is a step in exposure analysis in the MRA Protocol.

odds ratio          (ACRONYM: OR)
  1. A relative measure of the difference in exposure between the diseased (cases) and not diseased (controls) individuals in a case-control study.  The OR is interpreted similarly to the relative risk.  (EPA 2003)
  2. A measure of association that quantifies the relationship between an exposure and health outcome from a comparative study; also known as the cross-product ratio.  (CDC 2005)
  3. A measure of treatment effect that compares the probability of a type of outcome in the treatment group with the outcome of a control group, i.e., [Pt ÷ (1 - Pt)] [Pc ÷ (1 - Pc)].  For instance, if the results of a trial were that the probability of death in a control group was 25% and the probability of death in a treatment group was 10%, the odds ratio of survival would be [0.10 ÷ (1.0 - 0.10)] ÷ [(0.25 ÷ (1.0 - 0.25)] = 0.33.  (NLM/NICHSR 2004)
outbreak
  1. 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.  (CDC 2005)

An epidemic limited to localized increase in the incidence of a disease, e.g., in a village, town, or closed institution.  (MERREA 2005)
RELATED TERMS: epidemic

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outbreak data

CDC data on foodborne disease outbreaks define an outbreak as an incident in which two or more persons experienced a similar illness after ingestion of a common food, and epidemiologic analysis implicated a food as the source of the illness.  There are two exceptions, botulism and chemical poisoning, in which one case constitutes an outbreak.  (USDA 2004)

outbreak, foodborne

An incident in which two or more cases of a similar illness result from eating the same food.  (FDA/CFSAN 2001)

pandemic
  1. A widespread epidemic throughout an area, nation or the world.  (EPA 2005b)
  2. An epidemic that affects a wide geographic area.  (CancerWeb 2005)
  3. An epidemic occurring over a very wide area (several countries or continents) and usually affecting a large proportion of the population.  (CDC 2005)
  4. An epidemic occurring worldwide, or over a very wide area, crossing international boundaries, and usually affecting a large number of people.  (MERREA 2005)

Denoting a disease affecting or attacking the population of an extensive region, country, continent, global; extensively epidemic.  (Stedman 2005)
RELATED TERMS: epidemic

person-time

A unit of measurement combining persons and time, used as denominator in instantaneous incidence rates.  It is the sum of individual units of time that the persons in the study population have been exposed to the condition of interest.  A variant is person-distance, e.g., as in passenger-miles.  The most frequently used person-time is person-years.  With this approach, each subject contributes only as many years of observation to the population at risk as he is actually observed; if he leaves after one year, he contributes one person-year; if after ten, ten person-years.  The method can be used to measure incidence over extended and variable tiffs periods.  (Last 1983)

person-year

The sum of the number of years each person in the study population is at risk; a metric used to aggregate the total population at risk assuming that 10 people at risk for one year is equivalent to 1 person at risk for 10 years.  (RAIS 2004, SRA 2004)

population risk

Population risk refers to an estimate of the extent of harm for the population or population segment being addressed.  It often refers to an analysis of the number of people living at a particular risk or hazard level.  (EPA 2004)

premature death

A death that occurs before statistical expectation, usually attributable to a specific cause, and usually referring to deaths statistically estimated in a population rather than to individuals.  (RAIS 2004, SRA 2004)

prevalence
  1. The proportion of disease cases that exist within a population at a specific point in time, relative to the number of individuals within that population at the same point in time.  (EPA 2003)
  2. The number of existing disease cases in a defined population during a specific time period.  (ATSDR 2004)
  3. The number or proportion of cases or events or conditions in a given population.  (CDC 2005)
  4. The number of events, e.g., instances of a given disease or other condition, in a given population at a designated time.  (MERREA 2005)
  5. The number of people in a population with a specific disease or condition at a given time, usually expressed as a ratio of the number of affected people to the total population.  (NLM/NICHSR 2004)
  6. The number of existing cases in a population who have the disease at a given point (or during a given period) of time.  (RAIS 2004, SRA 2004)

The total number of cases of a given disease at a particular point in time, includes new (i.e., incidence) as well as chronic cases.  (USDA 2004)
RELATED TERMS: contrast with incidence

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.  (CDC 2005)

prevalence survey

The measure of the current level of disease (s) or symptoms and exposures through a questionnaire that collects self-reported information from a defined population.  (ATSDR 2004)
RELATED TERMS: survey
Prevalence surveys can also include diagnostic testing (often serology) to assess disease/infection status; although most also include a questionnaire as a component.  Prevalence surveys in wild animal populations obviously do not include questionnaires.

primary contact

Person(s) in direct contact or associated with a communicable disease case.  (MERREA 2005)
RELATED TERMS: contact, direct contact

primary transmission

Direct or indirect transfer of a food- or waterborne pathogen from a contaminated medium to a susceptible host, whether or not disease is produced.  (ILSI 2000)

proportionate mortality ratio          (ACRONYM: PMR)
  1. The proportion of deaths due to the disease of interest in the exposed population divided by the proportion of deaths due to the disease of interest in the unexposed or reference population.  It is frequently converted to a percent by multiplying the ratio by 100.  (EPA 2003)
  2. The fraction of all deaths from a given cause in the study population divided by the same fraction from a standard population.  A tool for investigating cause-specific risks when only data on deaths are available.  If data on the population at risk are also available, standardized mortality ratios are preferred.  (RAIS 2004, SRA 2004)
prospective epidemiologic study

SEE: prospective study

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prospective study
  1. An epidemiologic study comparing those with an exposure of interest to those without the exposure.  These two cohorts are then followed over time to determine the differences in the rates of disease between the exposure subjects.  (EPA 2003)
  2. A study in which the investigators plan and manage the intervention of interest in selected groups of patients.  As such, investigators do not know what the outcomes will be when they undertake the study.  (NLM/NICHSR 2004)
  3. A study in which the disease events to be measured have not occurred when the study begins, and so study participants have no foreknowledge of their possible involvement.  Both follow-up studies and case-control studies can be prospective with respect to their accumulation of cases.  (NZ 2002)

An inquiry in which groups of individuals are selected in terms of whether they are or are not exposed to certain factors, and then followed over time to determine differences in the rate at which disease develops in relation to exposure to the factor.  Also called cohort study.  (RAIS 2004, SRA 2004)
RELATED TERMS: cohort study; contrast with retrospective study

public health surveillance
  1. The ongoing, systematic collection, analysis, and interpretation of health data.  This activity also involves timely dissemination of the data and use for public health programs.  (ATSDR 2004)
  2. 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.  (CDC 2005)

The systematic collection, analysis and interpretation of the health data that is used to plan, implement, and evaluate public health programs.  Also used to determine the need for public health action.  (MERREA 2005)
RELATED TERMS: surveillance, disease surveillance

rate
  1. An expression of the frequency with which an event occurs in a defined population.  (CDC 2005)
  2. In epidemiology,”rate” has special usage; it is the frequency with which an event occurs in a defined population, at or over a specified period of time.  A rate is therefore a ratio, and includes proportions.  (NZ 2002)
reference group

SEE: control group

respondent error
  1. A type of non-sampling error caused by respondents intentionally or unintentionally providing incorrect answers to research questions.  (ESOMAR 2001)

In surveys, a component of measurement error that results from the respondent deliberately or inadvertently answering incorrectly.  (NIST/SEMATECH 2005b)
RELATED TERMS: survey

retrospective study
  1. A kind of nonexperimental study in which all the phenomenon investigated occurs prior to the onset of the study.  Further, the samples of retrospective studies are usually chosen by the value the responses take. T his latter point creates special conceptual issues regarding causality, and the composition of comparison samples (see matches) is especially important.  Advantages of retrospective samples is that they allow one to investigate phenomena that are either unlikely or undesirable to occur in the future; further, since all key events occur in the past, retrospective studies can often be undertaken economically.  (NIST/SEMATECH 2005b)

A study in which investigators select groups of patients that have already been treated and analyze data from the events experienced by these patients.  These studies are subject to bias because investigators can select patient groups with known outcomes.  (NLM/NICHSR 2004)
RELATED TERMS: bias, cohort study, retrospective study; contrast with prospective study

secondary attack rate
  1. A measure of the frequency of new cases of a disease among the contacts of known cases.  (CDC 2005)

The number of cases of an infection that occur among contacts within the incubation period following exposure to a primary case in relation to the total number of exposed contacts.  (MERREA 2005)
RELATED TERMS: secondary transmission

secondary spread

SEE: secondary transmission

secondary transmission

Direct or indirect propagation of a pathogen from an infected person (with or without clinical illness) to additional people.  (ILSI 2000)

seroepidemiology

An epidemiology study or activity based on serologic testing of characteristic change in the serum level of specific antibodies.  Latent, subclinical infections and carrier states can thus be detected, in addition to clinically overt cases.  (MERREA 2004)

specificity

Specificity of a screening test is the proportion of truly nondiseased persons who are so identified by the screening test.  It is a measure of the probability of correctly identifying a nondiseased person with a screening test.  (Last 1983)
RELATED TERMS: sensitivity

standardized mortality ratio          (ACRONYM: SMR)
  1. This is the relative measure of the difference in risk between the exposed and unexposed populations in a cohort study.  The SMR is similar to the relative risk in both definition and interpretation.  This measure is usually standardized to control for any differences in age, sex, and/or race between the exposed and reference populations.  It is frequently converted to a percent by multiplying the ratio by 100.  (EPA 2003)
  2. The ratio of observed deaths in a population to the expected number of deaths as derived from rates in a standard population with adjustment of age and possibly other factors such as sex or race.  (RAIS 2004, SRA 2004)

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surveillance

Systematic ongoing collection, collation, and analysis of data and the timely dissemination of information to those who need to know so that action can be taken.  Surveillance is the essential feature of epidemiological practice.  (MERREA 2005)
RELATED TERMS: public health surveillance, disease surveillance

survey
  1. Surveys involve a (statistically) large number of interviews with respondents, using pre-designed questionnaires.  (ESOMAR 2001)
  2. A method of data collection that involves asking a fixed set of questions from selected individuals.  Key issues involve questionnaire development, (ideally random) sample selection, and nonresponse management.  (NIST/SEMATECH 2005b)
transmission of infection
  1. Any mode or mechanism by which an infectious agent is spread through the environment or to another person.  (CDC 2005)

Transmission of infectious agents.  Any mechanism by which an infectious agent is spread from a source or reservoir to another person.  Direct transmission is the direct and essentially immediate transfer of infectious agents to a receptive portal of entry through which human or animal infection may take place.  This may be by direct contact such as touching, kissing, biting, or sexual intercourse, or by the direct projection (droplet spread) of droplet spray onto the conjunctiva or onto the mucous membranes of the eyes, nose, or mouth.  It may also be by direct exposure of susceptible tissue to an agent in soil, compost, or decaying vegetable matter or by the bite of a rabid animal.  Indirect transmission is by vector or air; the latter is subdivided into droplet or dust.  (MERREA 2005)
RELATED TERMS: transmissible, secondary transmission

zoonoses
  1. An infectious disease that is transmissible under normal conditions from animals to humans.  (CDC 2005)
  2. Diseases and infections that are naturally transmitted between vertebrate animals and humans.  (CRCWQT 2002)
  3. Infections in animals that can be transmitted to humans.  (FDA 2001)
  4. A disease that can be passed from animals, whether wild or domesticated, to humans.  (MERREA 2005)
  5. Diseases of humans transmitted from animals.  (NZ 2002)
  6. A human disease that originates from an animal. (Jones 2006)

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