**M**

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

**N**

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

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

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

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

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

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

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

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

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

**NUMERATOR.**
The upper portion of a fraction.

**O**

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

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

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

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

**P**

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

**Q**

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

**R**

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

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

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

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

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

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

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

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

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

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

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

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

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

**S**

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

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

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

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

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

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

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

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

**SKEWED.**
A distribution that is asymmetrical.

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

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

**SPORADIC.**
A disease that occurs infrequently and irregularly.

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

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

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

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

**SURVEILLANCE.**
see PUBLIC HEALTH SURVEILLANCE

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

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

**T**

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

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

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

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

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

**U**

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

**V**

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

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

**VARIABLE.**
Any characteristic or attribute that can be measured.

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

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

**VIRULENCE.
**The degree of pathogenicity of a microorganism.

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

**Y**

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

**Z**

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