Direct Standardization
(Age-Adjusted Death Rates)
Lester R. Curtin, Ph.D. and Richard J. Klein, M.P.H.
Introduction
Most population-based mortality objectives and
subobjectives in Healthy People 2000 are tracked using
age-adjusted rates from the National Vital Statistics System
(appendix table I). The exceptions are deaths from
alcohol-related motor vehicle crashes, all motor vehicle
crashes, and work-related injuries (objectives 4.1, 9.3, and
10.1), which are monitored with crude death rates from other
data systems. In addition, objectives that refer to specific age
groups are tracked with age-specific rather than age-adjusted
rates.
Although the age-adjusted death rate (ADR) is one of
the most frequently used indexes of mortality, there is often
confusion concerning the basic concepts of its construction,
use, and interpretation. Some of the persistent issues include
the appropriateness of the ADR as a summary measure, the
validity of comparisons between ADRs, the method of
calculation, and the appropriateness of alternate summary
measures.
Why use age-adjusted death rates?
The total number of health events (for example, the
number of deaths) occurring in a population is useful for
determining the magnitude of a public health problem.
However, the absolute number of deaths is seldom useful for
comparisons between population groups (for example,
comparing males and females) or for comparing trends.
Assuming equal risk, a larger population group will tend to
generate more events (deaths) than a smaller group simply
because of its size. Therefore, to compare relative differences
in mortality among population groups, or for a given
population group over time, the number of deaths must be
related to the ‘‘population at risk’’ of dying to produce death
rates. The population of interest may be the entire population
of an area or a population subgroup (for example, people in
a certain age group).
The simplest death rate is the crude death rate (CDR),
defined as the total number of deaths divided by the midyear
population. CDRs are usually expressed as a rate per 1,000
or 100,000 population. CDRs for individual age cohorts,
called age-specific death rates (ASDRs), are the ratio of the
number of deaths in a given age group to the population of
that age group, again usually expressed per 1,000 or 100,000
population.
To compare the relative health of population groups or
to assess change in mortality over time, two criteria must be
considered. First, rates should relate the number of events to
the population at risk. Second, because many health
outcomes vary by age, the effect of the population’s age
distribution must be taken into account.
Although it does relate the number of events to the
population, the crude rate does not take into account the age
distribution of the population. As such, it is not an
appropriate measure for comparing differences between
population groups or for assessing change in mortality over
time. Because death rates for most diseases generally
increase with age, a population group with a relatively
Number 6—Revised
March 1995
From the CENTERS FOR DISEASE CONTROL AND PREVENTION/ National Center for Health Statistics
U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
Public Health Service
Centers for Disease Control and Prevention
National Center for Health Statistics CENTERS FOR DISEASE CONTROL
AND PREVENTION
Revised to clarify computations in tables III and V
