For example, the United States Census combines their population
data with crime data compiled from the FBI's Uniform Crime Reporting system (UCR)
to produce crime rates. These rates are calculated with a constant of 100,000
and are broken down by type of crime. As described by Sacco and Kennedy in The
Criminal Event, the constant in the above equation is designed so the crime rate
can be expressed per c (the constant) units of exposure. The selection of the
constant is largely arbitrary, but rates are traditionally expressed per 1,000
or per 100,000 units of exposure. It is, although, most important to choose a
value of c so the resulting rates will be meaningful to the reader. For example,
if you are calculating crime rates for a town of 10,000 people, and you choose a
value of c * 10,000, the results will be expressed in terms of a population
greater than the town. Consequently, the results will not be meaningful to the
town residents. On the other hand, if a value of 100 were chosen, a resident
reading the rates would see how many people in a group of 100 would be likely to
be a victim of a given crime. In contrast to the constant, which is arbitrary,
and the numerator which is provided by observations and reports, the denominator
must be carefully determined for each type of crime. To determine this
denominator the target population must be established. The target population can
be defined as "the population we want to talk about" (Mosteller,
Fienberg, and Rourke, 1983). "The usual practice is to use the total
population residing within the jurisdiction in which the count of events has
taken place" (Sacco and Kennedy, 1996). Although, this method does not take
into account "the demographic characteristics of the population or the
empirical fact that different segments of the population are involved in
criminal events with different levels of frequency" (Sacco and Kennedy,
1996). Segments of the population can be created by looking at cross sections of
age, sex and potential for victimization. Members of these segments may have a
higher probability to being exposed to a particular crime then members of other
segments. For example, people who spend a large amount of time outside their
homes in the evening may have a higher potential of being a victim of a crime
such as aggravated assault. If the entire population were used in the
denominator, the calculated rate of aggravated assaults would be artificially
high for the general population and artificially low for the segment of people
who go out in the evening. In summary, when reviewing crime rates, the value of
the denominator should be considered when drawing conclusions. If the
denominator reflects the size of the total population residing within the
measured jurisdiction, conclusions should reflect the possibility that subsets
of the population may be incorrectly included in the calculated crime rate. If a
subset such as males aged 16 - 30, were used in the calculations, conclusions
should consider the subset of the population which may be excluded.