Computation and Measurement of Risk - Mathematics and Statistics for the Environment - page 195

100,000 population. In the United States in 2003, a total of 108,256 deaths were attributed to acci-
dents (unintentional injuries), yielding a cause-specific mortality rate of 37.2 per 100,000 population
(WISQARS, 2012).

8.4.1.3 Age-Specific Mortality Rate
An age-specific mortality rate is a mortality rate limited to a particular age group. The numerator is
the number of deaths in that age group; the denominator is the number of persons in that age group
in the population. In the United States in 2003, a total of 130,761 deaths occurred among persons
ages 25 to 44 years, for an age-specific mortality rate of 153.0 per 100,000 25- to 44-year-olds
(WISQARS, 2012). Some specific types of age-specific mortality rates are neonatal, postneonatal,
and infant mortality rates, as described in the following sections.

8.4.1.4 Infant Mortality Rate
The infant mortality rate is perhaps the most commonly used measure for comparing health status
among nations. It is calculated as follows:

Number of deaths among children<1year of agereported duringagiventimeperiod
Number of live births reported duringthe same time period

×10 n

The infant mortality rate is generally calculated on an annual basis. It is a widely used measure of
health status because it reflects the health of the mother and infant during pregnancy and the year
thereafter. The health of the mother and infant, in turn, reflects a wide variety of factors, includ-
ing access to prenatal care, prevalence of prenatal maternal health behaviors (such as alcohol or
tobacco use and proper nutrition during pregnancy), postnatal care and behaviors (including child-
hood immunizations and proper nutrition), sanitation, and infection control.
Is the infant mortality rate a ratio? Yes. Is it a proportion? No, because some of the deaths in the
numerator occurred among children born the previous year. Consider the infant mortality rate in
2003. That year, 28,025 infants died and 4,089,950 children were born, for an infant mortality rate
of 6951 per 1000. Undoubtedly, some of the deaths in 2003 occurred among children born in 2002,
but the denominator includes only children born in 2003.
Is the infant mortality rate truly a rate? No, because the denominator is not the size of the mid-
year population of children less than 1 year of age in 2003. In fact, the age-specific death rate for
children less than 1 year of age for 2003 was 694.7 per 100,000 (WISQARS, 2012). Obviously, the
infant mortality rate and the age-specific death rate for infants are very similar (695.1 vs. 694.7 per
100,000) and close enough for most purposes. They are not exactly the same, however, because the
estimated number of infants residing in the United States on July 1, 2003, was slightly larger than
the number of children born in the United States in 2002, presumably because of immigration.

8.4.1.5 Neonatal Mortality Rate
The neonatal period covers birth up to but not including 28 days. The numerator of the neonatal
mortality rate therefore is the number of deaths among children under 28 days of age during a given
time period. The denominator of the neonatal mortality rate, like that of the infant mortality rate,
is the number of live births reported during the same time period. The neonatal mortality rate is
usually expressed per 1000 live births. In 2003, the neonatal mortality rate in the United States was
4.7 per 1000 live births (WISQARS, 2012).

8.4.1.6 Postneonatal Mortality Rate
The postneonatal period is defined as the period from 28 days of age up to but not including 1
year of age. The numerator of the postneonatal mortality rate therefore is the number of deaths
among children from 28 days up to but not including 1 year of age during a given time period. The