WS - E - Conducting Community Surveys

representative of the population, and we are not able to measure how well our sample results reflect the whole population. A probability sample is one that is selected in such a way that each member of the population's sampling frame has a known probability of being selected and that probability is greater than zero. Note that this does not necessarily mean an equal probability of being selected, though some probability samples have that characteristic. In some circumstances it is desirable to select some members of population with certainty. If you are using something other than equal probability of selection, weights should be used to take into account the differing sampling probabilities. A non-probability sample is a sample in which you either do not know the chances that each member of the population has of being selected, or there are some members of the population that will not be selected. Interviewing the women in your office about their theft victimization experiences might help you think about the problem of women as victims of theft, but this group is unlikely to be representative of the larger population in your city. When using a probability sample, two things heavily influence the confidence in your estimates. The first is sample size. This is the number of cases in your sample. The bigger your sample, the greater the confidence that your statistic is an accurate measure of the population parameter. You have some control over your sample size, at least within the bounds of your budget or the budget of the agency funding your research. The second factor which influences confidence in estimates is the prevalence of the characteristics of interest in the population. The greater the prevalence, the smaller the sample you will need to get the same level of confidence. The more rare the characteristic is in the population, the larger the sample required to get the same level of confidence. While you have no control over the prevalence in your jurisdiction, your knowledge about how common the characteristic is in your population can be used to design your sampling strategy. Simple random sampling is one of four common probability sampling designs. Each has its advantages and disadvantages. Simple random sampling is the type of sampling design we usually think about, and the sampling strategy which is recommended in software package included with this guide. Once the sample size has been determined and the sampling frame has been developed by inputting the jurisdiction's telephone prefixes to generate a list of telephone numbers, each item on the list is given a unique number. Using the software's capacity to produce random numbers, cases (telephone numbers) are selected for the sample. Each case (telephone number) in the sampling frame (list of telephone numbers) has exactly the same probability of selection and that probability is equal to the sample size divided by the population size (number of entries in the sampling frame). It is important to include all prefixes that 12 A Practical Guide for Police