Clinical Epidemiology & EvidenceBased Medicine
Glossary:
Terminology Specific to Epidemiology
Updated
July 02, 2005
Section Contents:
Disease,
Outcome and Factor Measures:
 Proportion: A dimensionless number between 0.0 an 1.0 (if a
probability) or, equivalently, between 0% and 100% (if a percentage) consisting of one
count as the numerator divided by another count as the denominator. Note that for
consistent, unbiased interpretation, 1) all the individuals in the numerator must also be
included in the denominator, 2) each individual in the denominator must be at risk of
being in the numerator, and 3) all the individuals at risk of being in the numerator in a
group must be in the denominator. Equivalently, the probability that an atrisk individual
will acquire a condition. Point prevalence is a proportion. Proportions are often
missidentified as "rates" (e.g. casefatality "rate", attack
"rate", pregnancy "rate", relapse "rate"). (Note: Some
introductory texts mislabel these proportions as "rates".)
 Rate: An instantaneous or "velocity" measure that can range from 0.0 to
infinity, has the dimensions of number of individuals per group  unit of time (e.g., 2.5
cases per dogmonth), and is the number of individuals in the atrisk group that
experience the event during one time unit (per hour, day, week, month, year, ...). A rate
is a ratio of the number of events in a group of individuals at risk for the event divided
by the total time units contributed by the individuals atrisk of the event and is not a
proportion. Proportions are often missidentified as "rates".
 Ratio: A numerator divided by a denominator that usually does not include subjects
of the numerator and is not restricted to values between 0.0 and 1.0 as are proportions.
 Incidence Rate: The rate of onset of a disease in a population per unit time
calculated as the number of new cases in a population divided by the total time units each
individual in the population was observed before either disease onset occurred in the
individual or observation of the individual ceased. Theoretically, incidence is an
instantaneous rate.
 Cumulative Incidence: The proportion of a fixed population that become diseased
within the stated time period (e.g. month, year); not a rate but often referred to as such
(e.g. the annual incidence "rate" is actually the cumulative annual incidence, a
proportion).
 Attack "Rate": The proportion of susceptible individuals exposed to a
specific risk factor in a disease outbreak that become cases. For an infectious risk
factor, the attack rate is the number of secondary cases occurring within the accepted
incubation period divided by the number of susceptible individuals in a closed group
exposed to the primary (index) case.
 Case Fatality "Rate": Cumulative incidence of death in the group of
individuals that develop the disease over a time period (often unstated); a proportion,
not a rate.
 Mortality "Rate": The proportion of individuals in a population that die
in a given period of time, usually a year and usually multiplied by a 10^{n}
population size so it is expressed as the number per 1,000, 10,000, 100,000, ...
individuals per year. These proportions are often broken into causespecific and
agespecific proportions and are often standardized so different groups can be compared
and the population at the middle of the time interval is often used as the denominator.
 Prevalence (Point) (Pr): In the clinical setting, prevalence is the clinician’s
estimate of the probability that an individual has a given disease, based on what the
clinician knows to that point (e.g., history, physical exam), before doing a diagnostic
test. In the population sense, prevalence is the probability at a specific point in time
that an individual randomly selected from a group will have the condition, which is
equivalent to the proportion of animals in the group that have the disease. Group
prevalence is calculated by dividing the number of individuals in a group that have this
disease by the total number of individuals in the group that are at risk of the disease.
Note that prevalence is a good measure of the amount of a chronic, low mortality disease
in a population, but is not a good measure of the amount of short duration or high
fatality disease. Prevalence is often established by crosssectional surveys. However,
note that the prevalence of test positives in a survey is equivalent to the actual disease
prevalence only if the test used is a perfect test.
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Risk:
 Risk: Risk is the likelihood, usually quantified as an incidence rate
or cumulative incidence proportion, that an individual will develop a given disease in a
given time period.
 Risk Factor (Condition Determinant, Predisposing Factor): An individual attribute
or exposure that is positively or negatively associated with the occurrence of a
disease.
 Attribute: Risk factor that is an intrinsic characteristic of the individual (e.g.,
genetic susceptibility, age, sex, breed, weight).
 Exposure: Risk factor that is in the environment external to the individual (e.g.,
nutrition, housing, husbandry practice, or toxic agent).
 Competing Risks: Other sets of risk factors than can cause the condition of interest
which coexist with the set of factors of interest, that is; those things that cause
"red herring" cases in outbreak investigations.
 Induction Period: Time between exposure to a specific risk factor and the initiation
of the disease. Generally the longer the induction period, the more difficult is the
assessment of the association between the risk factor(s) and the disease and thus the
evaluation of causality.
 Latent Period: Time between biologic onset of disease and disease detection
(clinical  appearance of clinical signs or subclinical  positive diagnostic tests).
 Risk (Key) Determinant: A term applied to risk factors that a veterinarian can
modify or eliminate in a specific situation to prevent or correct the disease.
 Risk Marker: A noncausal factor associated sufficiently well with a risk factor
that it can be used as a reliable marker, or indicator, of the risk factor’s
presence.
 Risk Measures:
 Attributable Risk (AR): The risk in the group exposed to a risk factor minus the
risk in the group not exposed to that risk factor. The underlying or background risk
without that exposure is usually assumed to be the same in both groups.
 Etiologic Fraction: (Population attributable risk) The proportion of all cases of a
disease that are attributable to an exposure or risk. This is the proportion of the
disease in the population that would be eliminated if that exposure were eliminated or
prevented.
 Relative Risk (RR): A ratio ranging from 0 to infinity that indicates the strength
of the association between the risk factor and the disease outcome and is calculated by
dividing the risk in the group exposed to a risk factor by the risk in the unexposed
group. A RR value statistically significantly larger than 1 indicates the exposure is
associated with increased risk of disease, a RR value not statistically significantly
different from 1 indicates there is no association between the exposure and the risk of
disease, and a RR value statistically significantly less than 1 indicates the exposure is
associated with decreased risk of disease; that is, the exposure is protective.
 Exposure Odds Ratio (OR): An estimate of relative risk that is obtained from a
casecontrol study and that is similar to the relative risk when the disease is relatively
rare (a cumulative annual incidence of < 5% in the unexposed population). Otherwise, it
overestimates relative risk. The oddsratio is interpreted in the same fashion as
relative risk.
 Number Needed to Treat (NNT): NNT is the number of individuals a clinician would
need to treat to prevent one adverse outcome in that group of similar individuals at risk
of the problem. This measure establishes the benefit of an intervention compared to doing
nothing against a disease in individuals at risk of that disease when adverse events would
still be expected even with the intervention (e.g. daily aspirin to prevent myocardial
infarction). NNT is the reciprocal of the attributable risk or the reciprocal of the
difference between the proportions of treated and nontreated individuals experiencing
events over some period of time.
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