WS - E - Conducting Community Surveys

significantly cover the area of interest. Leaving out some will lessen the representativeness of the sample. Simple random sampling is most useful when you are not investigating rare events or small population segments. For example, if you think that only about 2 percent of the female undergraduates at a certain college have been the victim of a theft in the prior six months, then simple random sampling is a very costly approach. To be sure that we have 30 women who have been victims of theft in our sample, we would have to interview 1,500 female students. In contrast, if 40 percent of female undergraduates are likely to have been theft victims, then simple random sampling may be quite efficient. This, of course, is the difficulty for a city-wide sample for victimization; despite prominence in the media, violent crime events are relatively rare in a jurisdiction. Very few persons are victimized by violent crime. Generating the sampling frame Now that we've discussed the issues underlying determining sample size, we’re back to the question & How many are enough? Since violent crime rates & that is, rates per 1,000 population & are fairly well established and consistent across different places, we know that violent crime is a relatively rare event. Using the CVS software, the sampling frame for the CVS and community policing survey consists of randomly generated telephone numbers which, by and large, represent separate households. (Some may represent multiple phones within a single household while others serve multiple households, such as a rooming house.) This method of telephone number generation is known as random-digit dialing or RDD. In developing a sampling frame, RDD avoids the limitations of using phone books, since many telephone numbers are unlisted. In generating the sampling frame, enough numbers must be generated to account for refusals & that is, the prospective respondents who decline to be interviewed & and ineligibles such as non-residential numbers, non-working numbers, facsimile, computer lines, and cell phones and numbers which duplicate other lines within a household. These ineligible numbers may constitute a very large portion of the sampling frame, particularly in moderate to high-income neighborhoods. There is no reliable information about ineligibles generated through RDD. An estimate of about 60 percent may be appropriate for purposes of planning. Actual experience will probably vary a great deal by location and depend on the effort applied to obtaining participation from the survey respondents. What is the appropriate sample size? Deciding on an appropriate sample size requires effort and planning. There are no standardized sample sizes for surveys. Recommended sample sizes are even independent of city size. Since violent crime is a relatively rare event and is Conducting Community Surveys 13