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Chapter 8: Selecting a Sample Introduction The purpose of most research is to learn something about some large group, called the population. In the Sudan Fertility Survey, for example, the objective was to learn about the fertility for the population of all married women living in northern Sudan. One way to learn something about a population is to collect data from all the members of the population. This is known as taking a census or enumerating the population. Generally, because of the time and cost involved, it is impractical to enumerate a population. The only practical alternative is to obtain data from a part of the population, called a sample. In designing your research you will have to decide whether to enumerate the population you are studying or to use a sample. Enumeration versus sampling Enumeration has one great advantage over sampling. For smaller populations, enumeration produces accurate descriptions of variables.If you collected data on absence from work for all workers in a government ministry for a certain period of time and every worker gave valid, accurate responses, the results would accurately describe absenteeism for that population of workers. However, if you used a sample, even with valid and reliable responses, the results for absenteeism would describe the rate of absence for the part of the population from whom you obtained data. Another sample would produce a different rate of absenteeism because every sample contains a certain error based on the variations that occur from one sample to the next. In Chapter 19 we explain why this occurs. With this apparent disadvantage, why is most social research based on samples? Why sample? The main reason for sampling is that many populations are simply too large to enumerate. Collecting data from a population of thousands or millions requires a huge planning effort and a large staff of interviewers, supervisors, and data processing personnel. Months or even years would be required and would be expensive. With a sample, data can be obtained in a short period of time and at a reasonable cost. All the planning and administrative requirements are greatly reduced as well. Similar practical considerations will probably determine whether you enumerate or use a sample. If the population you are studying is rather small and you can collect data from all its members relatively easily, by all means do so. Then, you will not have to deal with the problem of sampling error, which we discuss, in Chapter 19. However, if your population is too large to enumerate, you will have to use a sample. Sampling is not limited to selection of individuals to become respondents in a survey. Sampling can be used to select organizations, villages, other geographical or political entities, the content of newspapers for analysis, or time periods for observing certain behaviors. In fact, sampling can be applied to almost any unit of analysis you might think of. Assuming you decide to sample, you are then faced with a second choice. As suggested by the "decision tree" in Figure 8.1, you have to decide:
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