Glossary

Accuracy: the extent to which a measurement or study result correctly represents the characteristic or relationship that is being assessed.

Acquired immunodeficiency syndrome (AIDS): a disease characterized by a marked reduction in CD4+ T lymphocytes and associated defects in immune response caused by the human immunodeficiency virus (HIV).

Active surveillance: a system of data collection in which those responsible for collecting the information go into the community under observation (typically defined by geographic boundaries) to gather data from various sources.

Acute: a disease of short duration.

Acute myelogenous leukemia (AML): a heterogeneous group of disorders, also known as acute nonlymphocytic leukemia, each of which involves the uncontrolled proliferation of primitive blood-forming cells.

Adjustment: a procedure for overall comparison of two or more populations in which background differences in the distribution of covariables are removed. (See also Standardization.)

Age adjustment: a procedure used to calculate summary rates for different populations in which underlying differences in the age distributions are removed. (See also Age standardization.)

Age-specific rate: a rate (usually incidence or mortality) for a particular age group.

Age standardization (direct): a procedure for obtaining a weighted average of age-specific rates in which the weights are selected on the basis of a standard age distribution (eg, the population of the United States in 1940).

Allele: an alternate form of a gene or a genetic locus that differs from other forms in its specific sequence of nucleotides; certain alleles may affect the structure and function of the corresponding protein coded for by that gene, in turn affecting the susceptibility to a particular condition.

Alpha error: see Type I error.

Alzheimer’s disease: the most common form of dementia in many populations, first described in 1907 by Alois Alzheimer; affected individuals have characteristic abnormalities in their brains, including neurofibrillary tangles and plaques with a protein fragment, β-amyloid, at their core.

Analytic epidemiology: activities related to the identification of possible determinants of disease occurrence.

Analytic study: a research investigation designed to test a hypothesis, often used in reference to a study of an exposure–disease association.

Anthrax: an infectious disease caused by a spore-forming bacterium Bacillus anthracis. The natural occurrence of this illness typically involves inhalation of spores from animal products. This agent was used as a weapon of bioterror in 2001 in the United States, resulting in 22 cases and 5 deaths.

Antibody: a protein, often produced in response to exposure to an antigen, that binds to the antigen and thereby stimulates its inactivation by the immune system.

Antigen: a protein, usually foreign in origin, that is capable of generating an immune response in a host animal.

Antigenic drift: mutation of a pathogen (eg, influenza A), such that the surface antigens differ from those of previously existing strains.

Apgar score: a system of evaluating the health status of a newborn using five indicators, each assigned a maximum of two points; the score is named after its originator, Dr. Virginia Apgar.

Arithmetic mean: see Mean.

Arteriosclerosis: hardening of the arteries.

Association: the extent to which the occurrence of two or more characteristics is linked either through a causal or noncausal relationship.

Asymptomatic persons: individuals who have a particular disease but do not manifest abnormalities of function, appearance, or sensation typically associated with that disease.

Atrophy: abnormal wasting of tissues, organs, or the entire body.

Attack rate: the proportion of persons within a population who develop a particular outcome within a specified period of time.

Attributable risk percent: the percentage of the overall risk of a disease outcome within exposed persons, related to the exposure of interest.

Autoimmune disorder: a disease state in which affected individuals produce antibodies against their own cells or tissues.

Benign: a mild illness; when applied to an abnormal growth of cells (viz, a neoplasm), it connotes a slowly progressing defect that is not invading adjacent tissues (in contrast to the rapid growth and invasive behavior of a malignant neoplasm).

Beta error: see Type II error.

Bias: a nonrandom error in a study that leads to a distorted result.

Biological marker: a measurable characteristic that helps to classify either level of exposure to a risk factor or susceptibility to (or presence of) a disease.

Bioterrorism: the threat of use of biological agents by individuals or groups in order to create fear or cause illness or death.

Birth cohort effect: an unusual age-specific rate (either incidence or mortality) within cross-sectional data that reflects the shared experience of persons born in specific years (birth cohort).

Blinding: assignment of treatment to individual subjects in a way such that subjects only (single blinding) or both subjects and treating physicians (double blinding) do not know the actual treatment allocation.

Borrelia burgdorferi: a spirochete that is borne by a particular deer tick that when transmitted to humans can cause Lyme disease (a systemic illness characterized by a skin rash, joint pains, and in advanced cases, cardiac and neurologic manifestations).

Botulism: a muscle-paralyzing disease caused by a toxin produced by the bacterium Clostridium botulinum. Natural occurrence typically results from consumption of contaminated food, but the toxin is now considered a potential agent of bioterrorism.

Bronchoscopic examination: the insertion of an instrument (viz, bronchoscope) to help visualize the trachea and bronchi and to facilitate the collection of specimens from these tissues.

Campylobacter jejuni: a species of the bacterial genus Campylobacter that is a leading cause of food-borne illness, most often arising in isolated events (rather than outbreaks), and characterized by fever, abdominal cramping, and diarrhea (occasionally bloody).

Cancer: a heterogeneous group of diseases characterized by the abnormal, uncontrolled growth of cells, which are capable of crossing normal anatomic boundaries to invade other tissues and even spread to remote anatomic sites.

Candidate region: a physical location on a chromosome believed to contain a potential disease susceptibility gene and identified by a genomewide scan and subsequent linkage analysis.

Case: a person who has a disease of interest. (See also Incident case and Prevalent case.)

Case–control study: an observational study in which subjects are sampled based on the presence (cases) or absence (controls) of the disease of interest. Information is collected about earlier exposure to risk factors of interest.

Case fatality: the proportion of persons with a particular disease who die from that disease within a specified period of time.

Causality: the extent to which the occurrence of a risk factor is responsible for the subsequent occurrence of a disease outcome.

Cerebral palsy: a disorder manifested by speech disturbances and lack of muscular coordination that arises from damage to the brain of a newborn before, during, or shortly after birth.

Cerebrovascular accident: a deficit in the delivery of oxygenated blood to the brain that may occur because of a blood clot or a hemorrhage; a synonym for a stroke.

Chance node: an element in a decision analysis that represents a point at which specified outcomes are determined on the basis of probability.

Cholesterol: a steroid that is abundant in animal tissues and is necessary for normal function; elevated levels of total cholesterol circulating in the blood of a host are associated with increased risks of cardiovascular disease.

Chronic: a disease of long duration.

Chronic obstructive pulmonary disease (COPD): an abnormal and long-standing reduction in airflow into and out of the lungs, typically caused either by chronic bronchitis or emphysema.

Clinical scenario: one of two or more alternative paths of management available in a decision analysis.

Clinical trial: an experimental study that is designed to compare the therapeutic benefits of two or more treatments.

Clostridium botulinum: the bacterium that produces a toxin responsible for causing botulism. Natural occurrence typically results from consumption of contaminated food, but the toxin is considered a potential agent of bioterrorism.

Clostridium perfringens: the bacterium responsible for a food-borne illness that often occurs in outbreaks and is characterized by diarrhea and abdominal cramping.

Cluster: a group of cases of a disease closely linked in time, place of occurrence, or both.

Cochrane Collaboration: an international organization dedicated to promoting well-informed health care decisions by preparing, maintaining, and ensuring accessibility to current, rigorous, systematic reviews of the benefits and risks of health care interventions. The organization is named in memory of Archie Cochrane, a physician epidemiologist who advocated using the best available evidence to guide health care decisions.

Coefficient of determination: the square of the correlation coefficient; it represents the proportion of total variability in an outcome that can be explained by the predictors in a regression model.

Cohort: a group of persons that shares a common attribute, such as birth in a particular year or residence in a particular town, and is followed over time.

Cohort study: an observational study in which subjects are sampled based on the presence (exposed) or absence (unexposed) of a risk factor of interest. These subjects are followed over time for the development of a disease outcome of interest. (See also Prospective cohort study and Retrospective cohort study.)

Common-source exposure: contact with a risk factor that originates in the shared environment of multiple persons.

Concordant results: the same outcome status for two or more individuals, as in a pair-matched case–control study in which both the case and the control are exposed (or unexposed).

Confidence interval: a range of values for a measure that is believed to contain the true value within a specified level (eg, 95%) of certainty.

Confounder: a variable that distorts the apparent relationship between an exposure and a disease of interest.

Confounding: a systematic error in a study that arises from mixing of the effect of the exposure of interest with other associated correlates of the disease outcome.

Control: in a case–control study, a subject without the disease of interest. (See also Adjustment.)

Control group: a population of comparison subjects in an analytic investigation.

Coronary artery disease: complete or partial blockage of the blood vessels that bring oxygenated blood to the heart muscle (myocardium), usually arising from atherosclerosis; if the reduction in blood flow is severe, a myocardial infarction may result.

Coronavirus: a family of viruses characterized by the appearance of a corona (crown) on electron microscopic imaging. These viruses are frequent causes of the common cold, and in the winter of 2002–2003, a new member of the family emerged as the cause of severe acute respiratory syndrome (SARS).

Correlation coefficient: a statistical measure of the relatedness of two variables; it can range from –1 (perfectly related inversely to each other) to +1 (perfectly related in the same direction to each other). When the variables are unrelated to each other, the correlation coefficient has a value of zero.

Correlation study: a hypothesis-generating investigation in which the values of two or more summary characteristics are associated across different population groups.

Cosegregation: the tendency of alleles on the same chromosome to be inherited together.

Cross-sectional study: an analytic investigation in which subjects are sampled at a fixed point or period of time, and the associations between the concurrent presence or absence of risk factors and diseases are then investigated.

Crude mortality rate: the rapidity with which persons within a given population die from a particular disease, without adjustment for the underlying age distribution of the population.

Cumulative incidence: the risk of developing a particular disease within a specified period of time.

Cutoff point: a value on an ordinal or a continuous scale of measurement used to distinguish categories. For example, values above this threshold may be classified as “abnormal” and values below this point may be classified as “normal.”

Death rate: see Mortality rate.

Decision analysis: a formal probabilistic process for making clinical decisions that incorporates information on medical options, anticipated likelihoods of various outcomes, and the uncertainty associated with clinical information.

Decision diagram: a flow chart used in decision analysis that identifies the clinical management choices, probabilities of events, and likelihoods of outcomes.

Decision node: an element of a decision tree that represents a choice between two or more competing alternative management approaches.

Decision tree: see Decision diagram.

Dementia: a condition characterized by impaired short- and long-term memory, along with disturbances of other cognitive functions, such as speech and the perception of spatial relationships.

Dependent variable: see Outcome variable.

Descriptive epidemiology: activities related to characterizing patterns of disease occurrence.

Diabetes mellitus: a disorder of carbohydrate regulation caused by either a markedly reduced or absent production of insulin by the pancreas (type I) or a decreased sensitivity to the effects of insulin in the peripheral tissues (type II).

Differential misclassification: incorrect categorization of the status of subjects with regard to one variable (eg, exposure) that is influenced by other characteristics of interest (eg, disease status).

Discordant results: different outcome status for two or more individuals, as in a pair-matched case–control study, when one subject in a pair is exposed and the other individual is unexposed to the risk factor of interest.

Disease outbreak: a sudden, unexpected increase in the occurrence of a disease within a relatively limited geographic area.

Diuresis: an abnormally elevated volume of urine production.

Dizygotic twin: fraternal twins resulting from the fertilization of two separate ova by two separate spermatozoa; the members of this pair are no more similar genetically than are two nontwin siblings.

Dose–response relationship: an exposure–disease association in which the risk of developing a disease varies with respect to the intensity or duration of exposure.

Ebola virus: a filovirus named after a river in the Democratic Republic of the Congo where it was first identified. It is responsible for a life-threatening illness characterized by fever, headache, joint and muscle pains, diarrhea, vomiting, and in some patients, external bleeding.

Eclampsia: the occurrence of one or more seizures that cannot be attributed to an underlying neurologic condition (such as epilepsy or a cerebral hemorrhage) in a patient with preeclampsia.

Ecologic fallacy: an association between summary characteristics across populations without actual linkage of the characteristics within individual persons.

Ecologic study: see Correlation study.

ELISA: enzyme-linked immunosorbent assay; it can be used to test for antibodies to an infectious agent.

Emerging infectious disease: an infection that has newly appeared within a population or has existed but is rapidly increasing in incidence or geographic range.

Empiric treatment: in the context of infectious illness, the initiation of an antibiotic treatment against a spectrum of suspected potential pathogens, in the absence of a documented specific pathogen(s).

Endemic rate: the usual rate of occurrence of particular events within a population.

Eosinophilia-myalgia syndrome: a condition characterized by muscle pains, and in some patients joint pains, skin thickening, hair loss, or intestinal disease, accompanied by an abnormally elevated level of eosinophil cells in the blood.

Epidemic: a dramatic increase above the usual or expected rate of occurrence of particular events within a population.

Epidemiology: the study of the distribution and determinants of disease within human populations.

Escherichia coli O157:H7: a strain of bacteria that is a cause of the hemolytic uremic syndrome, which occurs in humans who consume food products contaminated with this pathogen.

Etiology: the cause(s) of a disease or the study of disease causation.

Evidence-based medicine: the integration of current best evidence from research with clinical expertise, pathophysiologic knowledge, and patient preferences, used to make health care decisions.

Excess risk: the extra risk of the occurrence of a particular disease among persons exposed to a risk factor of interest. (See also Risk difference.)

Exclusions: persons who are eliminated from an analytic study because they do not satisfy the eligibility (inclusion) criteria.

Expected utility: a numerical value that represents the average result if the decision maker follows a particular path in a decision analysis.

Exposure: contact with or possession of a characteristic that is suspected to influence the risk of developing a particular disease.

External validity: the extent to which the conclusions of a study can be correctly applied to persons beyond those who were investigated. (See also Generalize.)

False-negative: a test result that is normal (negative) despite the true presence of the disease of interest or a study result that incorrectly fails to identify a true effect. (See also Type II error.)

False-positive: a test result that is abnormal (positive) despite the true absence of the disease of interest or a study result that incorrectly suggests an effect, when in truth, the purported effect does not exist. (See also Type I error.)

Familial aggregation: the extent to which the occurrence of a particular disease tends to cluster within families.

Fixed effects model: a statistical approach to combining information from multiple sources in which it is assumed that the investigated relationship is constant across sources and any differences in individual results are attributable entirely to random variation.

Follow-up study: see Cohort study.

Food-borne disease: an illness that is caused by the ingestion of food or food products, often arising from contamination of the food with microbes or other toxic materials.

Framingham Heart Study: a landmark prospective cohort study of risk factors for cardiovascular disease initiated in 1950 among residents of Framingham, Massachusetts.

Generalize: the ability to extrapolate study results from the study subjects to other persons who were not investigated.

Genetic epidemiology: the use of epidemiologic techniques to study hereditary determinants of disease in human populations.

Genetic (linkage) map: a type of map of the genome in which markers are identified on the chromosomes and the relative distances between these markers are estimated by the frequency with which the markers are inherited together.

Genome: the full complement of genes on all chromosomes.

Genomewide scan: an approach to localizing candidate regions for genes contributing to the susceptibility for a specific disease by analyzing the extent to which the disease occurs in members of affected families in association with markers at known locations throughout the genome.

Genotype: the genetic constitution of an individual; may be used in reference to the particular allele(s) present at one or more gene loci.

Glucosuria: an abnormally elevated level of glucose in the urine, as my occur in diabetes mellitus.

Granulocyte: a mature granular white blood cell, which includes neutrophils as well as other types of cells.

Granulocytopenia: a condition marked by an abnormally low number of granulocytes in the blood, and which may predispose the host to infection.

Hantavirus: a virus named for a river in South Korea, where human infection was first recognized; this pathogen is capable of causing a hemorrhagic fever and a separate pulmonary syndrome in infected human hosts.

Hematologic: of or relating to the blood or blood-forming tissues.

Heterogeneity: the statistical property of variation in an investigated relationship across individual studies or across subgroups within a particular study.

Historical cohort study: see Retrospective cohort study.

Historical controls: subjects in a clinical study who were previously treated with the standard therapy before the new experimental treatment was introduced.

HIV: see Human immunodeficiency virus.

Homogeneity: the statistical property of lack of variation of an investigated relationship across individual studies or across individual subgroups within a particular study.

Human immunodeficiency virus: the cause of the acquired immunodeficiency syndrome (AIDS) and other HIV-related disorders.

Hyperglycemia: an abnormally high level of glucose in the blood, as may occur in untreated patients with diabetes mellitus.

Hypertension: an abnormal elevation in blood pressure.

Hypoglycemia: an abnormally low level of glucose in the blood, as may result from an overly aggressive administration of insulin in patients with diabetes mellitus.

Hypothesis-generating study: an exploratory investigation designed to formulate questions that are evaluated in subsequent analytic studies.

Hypothesis-testing study: an analytic investigation in which one or more specific refutable suppositions are evaluated.

Hypoxia: an abnormally low level of oxygen in the arterial blood.

Immunity: a state in which a host is not susceptible to a particular infection or disease.

Inbreeding study: a study in which the degree of selective breeding among members of a particular group is assessed with respect to risk of developing a particular disease.

Incidence density: see Incidence rate.

Incidence rate: the rapidity with which new cases of a particular disease arise within a given population.

Incident case: a person who is newly diagnosed with a disease of interest.

Incubation period: the time interval between contact with a risk factor (often an infectious agent) and the first clinical evidence of the resulting illness.

Independent variable: a factor that is suspected to influence the outcome of an analytic study.

Index case: in a disease outbreak, the first affected individual to be identified; in genetics, see Proband.

Information (or observation) bias: a systematic error in a study that arises from the manner in which data are collected from participants.

Informed consent: the process of providing a patient with information about the risks and benefits of a proposed treatment plan and then securing the patient’s (or if the patient is a child, the guardian’s) agreement to undergo the planned intervention recognizing the risks and benefits.

Insulin: a peptide hormone produced in the pancreas and secreted into the blood, which delivers it to target organs to help regulate glucose utilization, protein synthesis, and formation and storage of lipids.

Intention to treat: analysis of the results of a clinical trial based on initial treatment assignment regardless of whether the subjects completed the full course of treatment.

Internal validity: the extent to which the conclusions of a study are correct for the subjects under investigation.

Ketosis: a condition characterized by the enhanced production of ketone bodies, as may occur in the metabolic abnormalities associated with diabetes mellitus.

Latent period: time between exposure to a risk factor and subsequent development of clinical manifestations of a particular disease.

Lead-time bias: apparent increase in the length of survival of patients with a disease as a result of earlier detection of the disease through the use of a screening procedure.

Length-biased sampling: preferential detection of less aggressive forms of a disease through the use of a screening procedure.

Life expectancy: the expected, or average, duration of life for persons in a particular population, under the assumption that current age-specific mortality patterns continue to apply.

Likelihood: the probability of the occurrence of a specified event.

Likelihood ratio: the probability of a particular test result for a person with the disease of interest divided by the probability of that test result for a person without the disease of interest.

Likelihood ratio for a negative test result: the probability of a negative test result for a person with the disease of interest divided by the probability of a negative test result for a person without the disease of interest.

Likelihood ratio for a positive test result: the probability of a positive test result for a person with the disease of interest divided by the probability of a positive test result for a person without the disease of interest.

Linkage: the proximity of multiple genes or genetic markers on the same chromosome, which is related to the probability that a certain combination of alleles at these sites will be inherited as a linkage group or haplotype.

Linkage analysis: a statistical technique used to identify candidate regions for genes based on the examination of the closeness of association between the inheritance within affected families of the condition of interest and markers at known locations throughout the genome.

Linkage disequilibrium: an excess or deficiency of certain combinations of alleles from genes or markers that are located on the same chromosome; alleles at tightly linked sites often are inherited together, and therefore linkage disequilibrium may help to identify the location of a particular susceptibility gene that is inherited as part of a linkage group with genes or markers at known locations within the genome.

Longitudinal study: see Cohort study.

Malignancy: the property of being malignant, often used interchangeably with the term cancer.

Malignant: a severe disease that is resistant to treatment (eg, severe hypertension); the term often is used in relation to the behavior of cancers.

Marker: in genetics, an identifiable physical location on a chromosome or DNA segment useful in mapping genes and in performing linkage analysis.

Matching: a procedure for sampling comparison subjects based on whether key attributes (ie, matching factors) are similar to those of subjects in the index group.

Mean: the arithmetic average of a distribution of values; calculated as the sum of the individual values divided by the number of observations.

Meconium: the first intestinal discharges of a newborn; if passed prior to delivery, it may serve as a sign of fetal distress, and if aspirated by the newborn, may give rise to acute pulmonary distress.

Median: a measure of central tendency of a distribution; calculated as the mid-point of the distribution when individual values are ordered from the smallest to the largest.

Median survival time: the duration of time from diagnosis to death that is exceeded by exactly 50% of subjects with a particular disease.

Medical outcome: see Outcome.

Meta-analysis: a statistical combination or integration of the results of several independent research studies that are considered to be combinable.

Metabolic acidosis: an abnormally high level of acid and low level of bicarbonate in the blood and other tissues resulting either from an accumulation of acids from metabolic processes (as in diabetes mellitus) or from an abnormally high loss of bases from the body (as in diarrhea or renal disease).

Misclassification bias: incorrect characterization of the status of subjects with regard to a study variable, leading to a distorted conclusion. (See also Information bias.)

Mode: a measure of central tendency of a distribution; it is the value that occurs most frequently within the distribution.

Monozygotic twin: genetically identical individuals arising from the division of a single fertilized ovum.

Morbidity: a state of illness produced by a disease.

Mortality: death, usually in reference to death caused by a particular disease (viz, cause-specific mortality).

Mortality rate: the rapidity with which persons within a given population die from a particular disease.

Mycobacterium: a genus of bacteria, a member of which is M tuberculosis, also known as tubercle bacillus, the pathogen responsible for tuberculosis in humans.

Myocardial infarction: a sudden diminution in the delivery of oxygenated blood to the heart muscle (viz, myocardium), most commonly caused by partial or complete blockage of one or more of the coronary arteries.

Natural history: the progression of a disease through successive stages, often used to describe the course of an illness for which no effective treatment is available.

Negative predictive value: the probability that a person with a negative (normal) test result actually does not have the disease of interest.

Neoplasm: a new growth that arises from the abnormal proliferation of cells; the proliferation may be benign or malignant (viz, cancer).

Nephropathy: a disorder of the kidney; among diabetics, the disorder arises because of damage to the small blood vessels of the kidney, which can lead to failure of the kidneys in an advanced stage.

Neutropenia: the presence of an abnormally low level of neutrophils in the blood, placing the host at increased susceptibility to infection.

Neutrophil: a mature white blood cell in the granulocyte series necessary for normal host defense responses.

Nondifferential misclassification: incorrect categorization of the status of subjects with regard to one variable (eg, exposure) that is unrelated to another characteristic of interest (eg, disease status).

Nosocomial infection: an illness caused by exposure to a pathogen during hospitalization of the host.

Notifiable disease: a disease for which regular, frequent, and timely information on individual cases is considered necessary for the prevention and control of the disease.

Null value: the point on the scale of a measure of association that corresponds to no association (eg, 1 for the risk ratio and the odds ratio and 0 for the risk difference and the attributable risk percent).

Observation bias: see Information bias.

Observational study: a nonexperimental analytic study in which the investigator monitors, but does not influence, the exposure status of individual subjects and their subsequent disease status.

Odds: the probability that a particular event will occur divided by the probability that the event will not occur.

Odds ratio: the odds of a particular exposure among persons with a specific disease divided by the corresponding odds of exposure among persons without the disease of interest.

Opportunistic infection: an illness caused by a microorganism that is capable of causing disease only in a host whose resistance is lowered below normal levels.

Outbreak: see Disease outbreak.

Outcome: clinical events that result from patient management decisions (eg, morbidity, complications, quality of life, or mortality).

Outcome variable: in an analytic study, the response of interest (eg, development of disease).

Pandemic: an elevated occurrence of a disease across a wide geographic area, affecting a substantial proportion of the population.

Passive surveillance: a system of data collection in which those responsible for collecting information rely upon voluntary reporting by other individuals or groups without entering the community to gather data.

Pathogen: an agent responsible for the development of a particular disease.

Pathophysiology: derangement of function associated with a disease process.

Pedigree: the family members of a proband, identified with respect to their biological relationship to the proband and whether they are known to have the disease of interest.

Penetrance: the proportion of individuals with a particular genotype that exhibits the same phenotype under similar environmental conditions.

Perinatal asphyxia: an abnormally reduced level of oxygenation of a fetus during labor and delivery, or shortly thereafter.

Person-time: a unit of measurement used in the estimation of rates that reflects the amount of time observed for persons at risk of a particular event.

Person-to-person spread: propagation of a disease within a population by transfer from an affected person to susceptible persons.

Person-years: a common unit for measuring person-time; one person-year corresponds to one person being followed for one year, or alternatively, two persons each followed for one half year, and so forth.

Person-years of life lost: a measure of total life expectancy lost within a particular population because of premature death.

Phenotype: a category or group to which an individual may be assigned on the basis of one or more characteristics observable clinically or by laboratory assessment that reflect genetic variation or gene–environment interactions.

Placebo: an inert substance.

Placebo effect: occurs when persons affected with a specific illness demonstrate clinical improvement when treated with an inert substance.

Plague: an infectious disease caused by the bacterium Yersinia pestis. It is naturally transmitted to humans by the bite of a rodent flea and is a potential agent for bioterrorism.

Polymerase chain reaction: a laboratory technique for rapidly synthesizing large quantities of a particular portion of genetic material.

Population at risk: persons who are susceptible to a particular disease but who are not yet affected.

Population-based study: an analytic study in which subjects are sampled from the general population.

Positive predictive value: the probability that a person with a positive (abnormal) test result actually has the disease of interest.

Posttest odds of disease: the estimated probability, after the administration of a diagnostic test, that a patient has the disease of interest divided by the probability that the patient does not have the disease of interest.

Posttest probability of disease: the estimated likelihood, after the administration of a diagnostic test, that a patient has the disease of interest.

Power: see Statistical power.

Precision: the extent to which a measurement is narrowly characterized. Statistical precision is inversely related to the variance of the measurement.

Predictor variable: see Independent variable.

Preeclampsia: the abnormal occurrence of hypertension accompanied by either an abnormal collection of fluid in body tissues or abnormally increased levels of protein in the urine, or both, due to pregnancy.

Premature death: a death that occurs earlier than would be expected in the absence of a particular disease.

Pretest odds of disease: the estimated probability, prior to the administration of a diagnostic test, that a patient has the disease of interest divided by the probability that the patient does not have the disease of interest.

Pretest probability of disease: the estimated likelihood, prior to the administration of a diagnostic test, that a patient has the disease of interest.

Prevalence: the proportion of persons in a given population that has a particular disease at a point or interval of time.

Prevalent case: a person who has a disease of interest that was diagnosed in the past.

Proband: the first affected individual who brings his or her family to the attention of a researcher or clinician for the purposes of medical care or investigation.

Prognosis: the predicted rate of progression of a disease process and its likely outcome(s).

Prognostic factor: an attribute anticipated to be related to the progression and outcome of a disease process.

Proportion: one quantity divided by another quantity in which the population in the numerator is a subset of the population in the denominator. The possible values of a proportion range from zero to one.

Prospective cohort study: a cohort study in which exposure status and subsequent occurrence of disease both occur after the onset of the investigation.

Publication bias: a distortion in conclusions derived from published studies because of the selective factors associated with the likelihood of publication, including whether the findings were positive and statistically significant, and the potential proprietary interests of sponsors.

Random effects model: a statistical approach to combining information from multiple sources in which it is assumed that the investigated relationship varies across individual sources, in addition to the influences of random variation in estimates.

Randomization: procedure for assigning treatments to patients by chance.

Rate: the rapidity with which health events such as new diagnoses or deaths occur. (See also Incidence rate and Mortality rate.)

Rate ratio: the rate of occurrence of a specified health event among persons exposed to a particular risk factor divided by the corresponding rate among unexposed persons.

Ratio: one quantity divided by another quantity, in which the population in the numerator is not a part of the population in the denominator. The possible values of a ratio range from zero to positive infinity.

Recurrence risk: in genetic epidemiology, the risk of developing a particular disease experienced by relatives of a subject with that disease.

Relapse: the return of the manifestations of a disease after a period of diminished manifestations.

Relative risk: see Risk ratio.

Reliability: the extent to which multiple measurements of a characteristic are in agreement.

Remission: elimination or reduction in the number or severity of the manifestations of a disease, which may be transient or permanent.

Response variable: see Outcome variable.

Retinopathy: a disorder of the retina of the eye; among diabetics the disorder arises from damage to the small blood vessels of the retina and can lead to blindness.

Retrospective cohort study: a cohort study in which exposure status and subsequent development of disease both occur prior to the onset of the investigation.

Risk: the probability that an event (eg, development of disease) will occur within a specific period of time.

Risk difference: the risk of the occurrence of a particular disease among persons exposed to a given risk factor minus the corresponding risk among unexposed persons.

Risk factor: an attribute or agent suspected to be related to the occurrence of a particular disease.

Risk ratio: the likelihood of the occurrence of a particular disease among persons exposed to a given risk factor divided by the corresponding likelihood among unexposed persons.

Salmonella enteritidis: one of two serotypes of the bacterium Salmonella that is a common cause of salmonellosis, a food-borne illness characterized by fever, abdominal cramping, and diarrhea (sometimes bloody).

Salmonella typhimurium: one of two serotypes of the bacterium Salmonella that is a common cause of salmonellosis, a food-borne illness characterized by fever, abdominal cramping, and diarrhea (sometimes bloody).

Sample: a subset of a target population that is chosen for investigation.

SARS: see severe acute respiratory syndrome.

SARS-associated coronavirus (SARS-CoV): the agent which first emerged in the winter of 2002–2003 and is responsible for causing severe acute respiratory syndrome (SARS).

Screening: the use of tests to detect the presence of a particular disease among asymptomatic persons prior to the time that the disease would be recognized through routine clinical methods.

SEER Program: The Surveillance, Epidemiology and End Results Program of the National Cancer Institute; it consists of 11 population-based cancer registries in various locations within the United States.

Segregation analysis: a complex statistical technique used to assess whether a particular disease has, at least in part, a genetic origin, and if so the most likely mode of inheritance.

Selection bias: a systematic error in a study that arises from the manner in which subjects are sampled.

Sensitivity: the probability that a person who actually has the disease of interest will have a positive (abnormal) test result.

Sensitivity analysis: (1) in systematic reviews, including meta-analyses, the evaluation of the pattern of results across subgroups of studies to characterize possible sources of heterogeneity and their respective influences on the overall summary effect; (2) in decision analysis, use of different values for an uncertain likelihood to determine whether the preferred course of action remains unchanged.

Sentinel case(s): the initial person(s) affected by a particular illness during an outbreak.

Seroconversion: change in a person’s status from not having evidence of infection (such as antibodies) in the serum to having such evidence.

Seronegative: absence of evidence of infection in a person’s serum (synonym: antibody negative).

Seropositive: presence of evidence of infection in a person’s serum (synonym: antibody positive).

Severe acute respiratory syndrome (SARS): a highly contagious and life-threatening respiratory illness that first emerged in the winter of 2002–2003 in China and rapidly spread throughout the world.

Shigella sonnei: one of four species of the bacterium Shigella and the most common cause of shigellosis, a food-borne illness characterized by diarrhea (often bloody), abdominal cramping, and fever.

Sib-pair analysis: statistical analysis for many genetic linkage studies attempting to locate susceptibility genes for a particular disease in which the fundamental unit of analysis is a pair of siblings.

Smallpox: an infectious disease caused by variola virus that was eradicated in 1977. The illness had a high case fatality and variola now is considered to be a potential agent for bioterrorism.

Specificity: the probability that a person who actually does not have the disease of interest will have a negative (normal) test result.

Squamous cell carcinoma: a malignant neoplasm (cancer) arising from stratified squamous epithelium, but that may also occur in sites in which glandular or columnar epithelium normally occur.

Standardization: an analytic procedure for obtaining a summary measure for a population by applying standard weights to the measures within subgroups of the population.

Statistical power: the ability of a study to detect a true effect of a specified magnitude. The statistical power corresponds to 1 (Type II error).

Statistical significance: the likelihood that a difference as large as or larger than that observed between study groups could have occurred by chance alone in a sample of the size investigated. Usually, the level of statistical significance is stated as a p value (eg, p < 0.05).

Stroke: a sudden derangement in function, as in sunstroke or heat stroke; often used in relation to a sudden neurologic deficit that occurs because of insufficient delivery of oxygenated blood to the brain, as may occur following a blood clot or hemorrhage.

Subacute: a rate of progression of a condition that is intermediate between acute and chronic.

Surveillance: ongoing observation of a population for rapid and accurate detection of changes in the occurrence of particular diseases.

Survival: the likelihood of remaining alive for a specified period of time after the diagnosis of a particular disease.

Systematic error: see Bias.

Systematic review: a synthesis of medical evidence on a topic, in which the synthesis has been prepared using strategies to minimize errors.

Terminal node: in a decision tree, an element that represents the outcome for a particular clinical scenario.

T lymphocyte: a white blood cell that is responsible for cell-mediated immunity in the host.

Transmission: the process by which a pathogen passes from one source of infection to a new host.

True-negative: a test result that is normal (negative) when the disease of interest is actually absent.

True-positive: a test result that is abnormal (positive) when the disease of interest is actually present.

Tuberculosis: an infectious illness caused by Mycobacterium tuberculosis, characterized by a brief initial illness; in a minority of cases, a chronic active illness, primarily affecting the lungs, will occur months to years following infection.

Tularemia: an infectious disease caused by the bacterium Francisella tularensis. Natural infection results from the bite of an infected tick, deerfly, or insect, but the bacterium also is now considered a potential agent of bioterrorism.

Tumor: a swelling that may occur from an inflammatory process or a benign or malignantneoplasm.

Twin study: a study of genetic susceptibility in which concordance for occurrence of a particular disease is compared between dizygotic (fraternal) twins and monozygotic (identical) twins, or between twins reared together versus apart.

Type I error: rejection of the null hypothesis when it is actually correct.

Type II error: failure to reject the null hypothesis when it is actually incorrect.

Underlying cause of death: (1) the disease or injury that initiated the train of morbid events leading directly to death, or (2) the circumstances of the accident or violence that resulted in fatal injury.

Utility: in decision analysis, a patient’s preference for one outcome over another, usually graded on a scale of zero, representing death, to one, representing perfect health.

Validity: the extent to which a measurement or a study result correctly represents the characteristics or relationship of interest.

Variability: the property of having a spread of values, which may arise from random sources (viz, the operation of chance) or from systematic influences (viz, bias).

Viral hemorrhagic fevers: a group of life-threatening illnesses caused by the filoviruses (eg, Ebola and Marburg) and arenaviruses (eg, Lassa and Machupo). Natural occurrence is geographically limited, but these viruses now are considered potential agents of bioterrorism.

Viremia: the presence of virus particles in the blood of a host.

Vital statistics: information concerning patterns of registered life events, such as births, marriages, divorces, and deaths.

Weighted average: a summary measure in which some of the component data values are assigned greater influence than others. For example, precision-based weighting is the calculation of a summary measure in which the relative influence of individual results is based on statistical confidence in the respective results.

West Nile virus: a member of the Flavivirus genus first identified in 1937 in the West Nile district of Uganda. The virus is passed between birds by mosquitoes, which in turn can infect humans through bites. The illness first appeared in the Western Hemisphere in 1999 and is characterized by fever, nausea, vomiting, headache, myalgia, and in rare instances leads to a severe neurologic disease.

Withdrawals: subjects who are initially included in a study but later voluntarily or involuntarily terminate participation.

Years of potential life lost (YPLL): a measure of total life lost to a particular age (eg, 75 years) within a population because of premature deaths.