Estimating the number of gangs and gang members for small cities and rural counties was a slightly more complex task. For these samples, the following steps were completed:

1. Calculate the mean for agencies responding either "yes" or "no" to the question regarding the number of gangs and gang members.

2. Multiply the total number of jurisdictions from which the sample was derived by the percentage of agencies responding either "yes" or "no" to the question regarding the number of gangs and gang members.

3. Multiply the product of step 2 by the mean calculated in step 1.

To estimate the number of gangs and gang members for large cities and suburban counties, the average or mean number of gangs and gang members per jurisdiction was calculated. These estimates were controlled for population by stratification of respondent agencies into population groups of 50,000 and by calculation of a mean for each population group. To acquire the most accurate mean, the survey designers selected 40 as the minimum number of agencies in each population group from which a mean could be derived. Setting the minimum at 40 allowed the inclusion of a large number and wide range of agencies in each population group. Population groups that did not include 40 or more agencies were expanded equally on the high and low ends in increments of 25,000 until 40 or more agencies were included. After the expansion, a mean was calculated and matched with each nonresponding agency within the corresponding population group (see appendix D).

As with estimating the numbers of gangs and gang members, estimating the number of homicides required extrapolation for both random samples and for nonrespondents in large cities and suburban counties. The survey asked respondents to report the number of homicides in their jurisdiction based on both member-based and motive-based definitions of gang crime. Member-based gang crime was defined as "crime in which a gang member(s) is either the perpetrator or the victim, regardless of the motive," and motive-based gang crime was defined as "crime committed by a gang member(s) in which the underlying reason is to further the interests and activities of the gang." Respondents were first asked to report the number of member-based homicides. The next question asked the respondent to report the number of motive-based homicides that were a subset of the member-based number.

The response rate to the question concerning type of homicide varied among respondents. Only 68 percent of respondents who reported gangs reported both types of homicides; 89 percent reported member-based homicides, and 70 percent reported motive-based homicides.

The estimated number of homicides in each area type during 1997 was determined by multiplying the average number of homicides per jurisdiction by the estimated number of jurisdictions reporting gangs in 1997. The estimate was calculated separately for each type of homicide. In other words, the average number of member-based homicides was calculated using the responses of all those who responded to the question concerning member-based homicides, regardless of whether they responded to the question on motive-based homicides. The same process was used to calculate the average number of motive-based homicides.

In several instances throughout the following analyses, the small number of observations made calculating reliable estimates and averages difficult. Therefore, summary data were not included for categories in which there were fewer than 20 observations, unless otherwise noted.