WS - E - Conducting Community Surveys

and a probability sample is an important procedure. So, a brief and very basic discussion about sampling procedures is necessary. Population and samples When you sample people, you select a portion or subset of everyone you are interested in, then make generalizations from the subset to all people of this type. The larger group is called a population while a sample is the subset or a portion of the population. If you were interested in the experiences of women who have been victims of theft, the population is all women. Since it is not possible to interview all women, you will want to interview a subset of women. The subset is your sample, and it is made up of cases. In the example so far, the cases are those women selected for interviewing. If possible, you will want to know something about the population. For example, what proportion of the population have been victims of theft in the prior six months? To answer this question with a sample, you need to be sure that the sample is representative of the population. Characteristics of the population are called parameters. In this example, the proportion of women in the population who have been sexually assaulted in the prior six months is a parameter. Though this number exists, it is unknown. You will need to estimate it from the sample. The estimate of the population parameter from the sample is called a statistic. If the sample is unrepresentative of the whole population, then the statistic you calculate is unlikely to be an accurate estimate of the population parameter you care about. For example, if you interview women at a women's crisis clinic, you may overestimate the proportion of women who have been victims of theft because it is quite likely that this sample or subgroup has a higher than average theft rate than women in the population. How do you get a representative sample from your population? All sampling begins with the translation of a defined population into a sampling frame. A sampling frame is either a list of possible cases or a procedure for finding eligible cases. It is from this sampling frame that you will select your cases. Ideally, everyone in the population should show up in the sampling frame. Say you were interested in the experience of college undergraduate women who have been victims of theft. There is one university in your jurisdiction, but there are too many female undergraduates to interview. The university might be able to provide you with a list of undergraduates for you to use as a sampling frame. Issues of probability sampling Probability sampling includes a variety of techniques that produce cost-effective data sets that are representative of the population from which they are drawn and yield information that quantitatively describes our confidence in how well the sample estimates describe population characteristics. Non-probability samples can be cheap or expensive, but we often have no basis for claiming they are Conducting Community Surveys 11