Understanding Crime Rates

The purpose of calculating rates is to make "raw" counts comparable across units of analysis. Counting how many criminal acts of a given type occur in an area is important, but in order to make decisions about the incidence of crime in that area, we need more information. Say, for example, that we know that there were 10 assaults in community A and 20 in community B this year. Does that mean community B is more dangerous? That is, did community B's residents have a higher chance of being assaulted this year? To answer that question correctly, we need to think about how many people there are in each community. What if community A is a small town and community B is a large city? That should affect our interpretation of the number of assaults.

Rates help us put absolute numbers in proper context, enabling comparisons between different geographic areas, among subgroups within the population, or across time. To calculate rates, we need to know how many events occurred but also how many individuals were "at risk" of experiencing the event. Continuing the example above, let's say that these two communities, A and B, have the following populations and assault counts:

	A	B
Population	25,000	100,000
Assaults	10	20

Which of the two communities has a higher rate of assaults? To figure that out, we calculate the assault rate by dividing the number of assaults by the number of people in the community. Because crimes are rare, it is a common practice to multiply the result by 1,000 and report the rates per 1,000 people for ease of interpretation.

Following this practice for community A, the rate per 1,000 people would be: Community B's rate per 1,000 people would be:

Therefore, although community B had more total assaults this year, the risk of being assaulted was two times higher in community A.

Just as in this example, calculating crime rates from the NCVS requires knowing not only how many crimes occurred in a given year but also the number at risk for victimization. However, when national estimates are derived from a sample, as with the NCVS, caution must be used when comparing one estimate to another estimate or when comparing estimates over time. Although one estimate may be larger than another, estimates based on a sample have some degree of sampling error. When the sampling error around an estimate is taken into account, the estimates that appear different may not be statistically different. For more information on sampling error and computing standard errors with the NCVS, see User's Guide to the National Crime Victimization Survey (NCVS) Generalized Variance Functions.