Computation and Measurement of Risk - Mathematics and Statistics for the Environment - page 185

8.2.3 r ate
In environmental health, a rate is a measure of the frequency with which an event occurs in a defined
population over a specified period of time. Because rates put disease frequency in the perspective of
the size of the population, rates are particularly useful for comparing disease frequency in different
locations, at different times, or among different groups of persons with potentially different sized
populations; that is, a rate is a measure of risk.
To a non-environmental health practitioner, rate means how fast something is happening or
going. The speedometer of a car indicates the car's speed or rate of travel in miles or kilometers per
hour. This rate is always reported per some unit of time. Some environmental practitioners restrict
use of the term rate to similar measures that are expressed per unit of time. For these environmental
practitioners, especially those practicing environmental health, a rate describes how quickly disease
occurs in a population—for example, 70 new cases of breast cancer per 1000 women per year. This
measure conveys a sense of the speed with which disease occurs in a population and seems to imply
that this pattern has occurred and will continue to occur for the foreseeable future. This rate is an
incidence rate , described later in Section 8.3.
Other environmental health professionals use the term more loosely, referring to proportions
with case counts in the numerator and size of population in the denominator as rates. Thus, an
attack rate is the proportion of the population that develops illness during an outbreak; for example,
20 of 130 persons developed diarrhea after attending a picnic. (An alternative and more accurate
phrase for attack rate is incidence proportion .) A prevalence rate is the proportion of the popula-
tion that has a health condition at a point in time; for example, 70 influenza patients being reported
in a county in March 2005. A case-fatality rate is the proportion of persons with the disease who
die from it—for example, one death due to meningitis among a county's population. All of these
measures are proportions, and none is expressed per units of time; therefore, these measures are not
considered “true” rates by some, although use of the terminology is widespread.
An example of rate is

Number of womeninState Awho died fromheartdiseasein 2009
Estimated numberofwomen living in StateAon July 1, 2009

Table 8.2 summarizes some of the common environmental health measures as ratios, propor-
tions, or rates.

TABLE 8.2
Environmental Health Measures Categorized as Ratio, Proportion, or Rate

Condition Ratio Proportion Rate

Morbidity (disease) Risk ratio (relative risk) Attack rate (incidence proportion) Person-time incidence rate

Rate ratio Secondary attack rate

Odds ratio Point prevalence

Period prevalence Attributable proportion

Mortality (death) Death-to-case ratio Proportionate mortality Crude mortality rate

Case-fatality rate
Cause-specific mortality rate
Age-specific mortality rate
Maternal mortality rate
Infant mortality rate

Natality (birth) — — Crude birth rate

Crude fertility rate