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Statistical Concepts
Used in Demographic and Epidemiologic Studies
Adjusted
rate: A rate that has taken into
account influences on a crude rate, such as differences in age composition of
the population.
Adjustment:
A summarizing procedure for a statistical measure in which the effects of
differences in composition of the populations being compared have been
minimized by statistical methods. Adjustment often is performed on rates or
relative risks, commonly because of differing age distributions in populations
that are being compared. Examples are adjustment by regression analysis and by
standardization. The mathematical procedure commonly used to adjust rates for
age differences is direct or indirect standardization.
Age-specific
rate: Number of events occurring to a
specified age group per 100,000 population of the specified age group (for
example, age-specific fertility rate, death rate, marriage rate, illiteracy
rate, school enrollment rate, etc.).
Average:
A measure of the central tendency in a distribution, indicated by such
statistics as the median and arithmetic mean; the distribution around the
average can be narrow or broad.
Average
daily census: The average number of
patients in a facility on any day divided by the number of days in the
reporting calendar year.
Average
length of stay: The average number of
days of service rendered to each inpatient before discharge from an institution
during a given period, which is typically a calendar year.
Case
fatality rate: The number of patients
dying from a specific condition, divided by the number developing the condition
per 1,000 affected persons. For hospital patients this statistic may be the
rate of patients dying prior to discharge from hospital.
Cause-specific
death rate: Number of deaths from a
specific cause per 100,000 population.
Confidence
interval: The range of values that
has a probability of including the true value for the population; a 95%
confidence interval includes the true value for the population 95% of the time.
Crude
rate: The rate of actual demographic
or vital event that is based on an entire population, generally the annual
births or deaths per 100,000 total population. Referred to as "crude rates"
since they do not take into account a population's age structure.
General
fertility rate (GFR): The number of
live births per 100,000 women age 15-44 years in a given year.
Infant
mortality: Cumulative number of
deaths to babies, born alive, before they reach their first birthday.
Life
table: Statistical tool typically
used to portray expectation of life at various ages. Also provides information
on numbers of individuals who survive to various ages, median age at death,
age-specific death rates, and the probability of dying at certain ages.
Mean:
The arithmetic average of a set of values. It is calculated as the sum of the
values divided by the number of values.
Median:
The value in an ordered set of values above and below which there are an equal
number of values; the 50th percentile.
Mode:
The most frequent number in a set of numbers.
Percentage
occupancy for staffed beds: The ratio
of the number of inpatient days of care to the number of bed days open during
the year for staffed beds, expressed as a percentage.
Percentage
occupancy for licensed beds: The
ratio of the number of inpatient days of care to the number of bed days open
during the year for licensed beds, expressed as a percentage.
Population
per facility: A measure relating the
number of people residing in a specifically defined geographic area (county,
region, or state) by the number of short term nonfederal hospitals in that
specified area.
Prevalence:
The number of existing cases of a disease or health condition in a population
at some designed time. Point
Prevalence is the number of person ill divided by total
number in the population group at a time point;
Period Prevalence is
the number of person ill divided by the average population during a time
period.
Probability:
A measure ranging from zero to one, based on the relative frequency of a
specific event occurring in a sequence of total trials of events.
Proportion:
A type of ratio in which the denominator includes the numerator; for example
the ratio of females to the total population.
Randomization:
Governed by chance, as opposed to deterministic. Allocation of individuals to
groups, e.g., for experimental and control regimens by chance selection.
Range:
The distance between the smallest and largest numbers in a set of numbers.
Rate:
The frequency of a demographic event in a specified period of time divided by
the population at risk of the event.
Rate
of natural increase: The difference
(±) between the crude birth rate and the crude death rate in a given population
per year. If all children born live to reproduce, any number of births greater
than 1 per biological parent can potentially result in population increase.
Ratio:
The relation of one population subgroup to another subgroup, or to the total
population; the denominator of a ratio may or may not include the numerator. If
the denominator includes the numerator, it is a special type of ratio known as
a proportion.
Reference
population: The standard against
which a population that is being studied can be compared.
Sex-ratio:
A measure relating the number of males to females in a population or age group.
Stable
population:
A population that has constant fertility and mortality rates, no migration, and
consequently a fixed age distribution and constant growth rate.
Standardization:
A set of techniques used to remove as far as possible the effects of
differences in age or other confounding variables, when comparing two or more
populations. For example, Age standardization, often called “age
adjustment”, is one of the key tools used to control for the changing age
distribution, and thereby to make meaningful comparison of vital rates over
time and between groups.
Stationary
population: A stable population that
has a zero growth rate with constant numbers of births and deaths each year.
Statistical
cut-off: Point in time by which
records of vital events for a specific year must be received in order to be
included in the statistical analyses for that year.
Statistical
significance: Used to evaluate the
likelihood that chance variability may be considered an explanation for
observed results. An appropriate mathematical test of statistical significance
is calculated to determine the "p value", which is the probability that the
observed results may be due to chance alone. If the p value is less than an
arbitrarily chosen value, commonly selected as 0.05, the findings are accepted
as statistically significant at the 5 percent level. This indicates there is
less than 5 percent probability that the observed results are due to chance
alone.
Total
fertility rate (TFR): The average
number of children a woman will have assuming that the current age-specific
birth rates will remain constant throughout her childbearing years (15-44).
References:
Alderson, Michael. An Introduction to Epidemiology. Great Britain :
MacMillan Press, 1976.
Haupt, Arthur and Thomas T. Kane. Population Handbook. Washington ,
D.C. : Population Reference Bureau, Inc., 1991.
Last, John M. A Dictionary of Epidemiology. Second Edition. New York
: Oxford Press, 1988.
Nam , Charles B. Understanding Population Change. Itasca: Peacock
Publishers, Inc. 1994.
Friis, Robert H and Thomas A. Sellers. Epidemiology for Public
Health Practice (2nd ed). Gaithersburg : Aspen Publication, 1999.
Last Updated:
10/10/2006.
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