WS - E - Conducting Community Surveys

Table 1. Best guess for sample size Best guess of Minimum sample size to find n cases with victimization percent of people victimized n=30 n=50 n=100 n=150 0.5% 6,000 10,000 100,000 300,000 1% 3,000 5,000 10,000 15,000 5% 600 1,000 2,000 3,000 10% 300 500 1,000 1,500 15% 200 333 666 1,000 20% 150 250 500 750 25% 120 200 400 600 somewhat equally as rare in large cities as in small cities, recommended sample sizes will not vary much based upon the total population of the city. In general, a final sample size of at least 200-250 will be the absolute minimum necessary for making inferences to the larger population. In the 12 large cities in which the CVS questions were tested, final samples consisted of approximately 800 households (in each city) to produce a sample sufficient for making inferences about victimization experience. These numbers were necessary to generate a sufficient number of victims of violent crime in the sample since violent crime victimization is statistically a rare event. In general, an appropriate sample size will fall somewhere between these points, depending of course on the objectives of the survey. If the police agency is not interested in violent crime, and desires only to collect information about property crime victims, a smaller sample may be used since property crime is statistically more common than violent crime. One way to think of selecting sample size is to develop a "best guess" of the level of victimization experience being sought in the sample. Using Table 1, say that you estimate that half of 1 percent of persons in your community experience the type of victimization you Reno, Nevada wish to examine. If your population is 500,000, this best guess of victimization suggests that about 2,500 Reno police conduct persons are victims. To find 30 of the 2,500 community surveys victimized persons, you will need to call or interview twice each year, and a at least 6,000 persons & your sample size is 6,000. sample size of 430 A minimum number of 30 cases is an absolute provides reliable requirement for large sample analysis with statistical information. In general, procedures normally used. the larger the sample size the greater the If you are interested in looking at subgroups such as accuracy in the survey race or gender by type of victimization, then doubling estimates, assuming the the sample size is appropriate. This requires that sample is increased in a your minimum number of cases increases random manner. proportionally as indicated in the top row of Table 1. 14 A Practical Guide for Police