Chapter 7 SAMPLING PROCEDURES IN RESEARCH

1 123 Scott M. Smith and Gerald S. Albaum, An Introduction to Marketing RESEARCH , 2010 Chapter 7 SAMPLING PROCEDURES IN RESEARCH Researchers must answer many questions when developing a project to collect information about a population of interest. Consider the following questions involving SAMPLING : Should we take a census (complete canvas) or a sample? What kind of sample should be taken? What size should the sample be? Answering these questions depends upon the application of statistical inference.

2 This Chapter first considers the selection of sample types and the process of sample planning. It then describes different kinds of samples. Finally, related to design, but a unique decision area in its own right, is the determination of the size of the sample. PLANNING THE SAMPLE Two broad objectives are fundamental to the use of samples in survey RESEARCH projects: Estimation of information about a population based on a sample from the population, Hypothesis testing to compare the relationships between data items for some selected population groups.

3 Each involves making inferences about a population on the basis of information from a sample. The precision and accuracy of project results are affected by the manner in which the sample has been chosen. However, as Exhibit illustrates, precision is only a reflection of SAMPLING error and confidence limits and has nothing to do with accuracy. Exhibit Precision versus Accuracy in SAMPLING There often is confusion between precision and accuracy in marketing RESEARCH . When a researcher speaks of SAMPLING error and the size of the confidence limits placed on an estimate, that researcher is speaking of precision and not accuracy.

4 Accuracy is affected by nonresponse bias, memory error, misunderstanding of questions, problematic definition of terms, and processing errors, and not by sample size (Semon, 2000). The researcher must constantly ask: Have we asked the right questions? and Have we measured what we want to measure properly and correctly? The difference between precision and accuracy is analogous to the difference between errors of degree and errors of kind. Accuracy, like errors of kind, are clearly more serious. If a researcher has made the correct measurements, precision will show the degree of error; but if the right measurements have not been taken, precision is probably meaningless (Semon, 2000).

5 The quality of a sample depends upon the quality of design and execution of a RESEARCH project at every stage of the process. 124 Scott M. Smith and Gerald S. Albaum, An Introduction to Marketing RESEARCH , 2010 Consequently, strict attention must be paid to the planning of the sample. It must also be recognized that sample planning is only one part of planning the total RESEARCH project. The process of selecting a sample follows the well-defined progression of steps shown in Figure , and will be discussed in turn.

6 Figure Steps in Sample Planning defining the Population The first step in sample panning is to define the population to be investigated. A population, also known as a universe, is defined as the totality of all units or elements (individuals, households, organizations, etc.) to which one desires to generalize study results. While seemingly an easy task, an imprecise RESEARCH problem definition often leads to an imprecise population definition. Specifying a population involves identifying which elements (in terms of kind) are included, as well as where and when.

7 For example, a group medical practice that is considering expanding into sports medicine might acquire information from any or all of the distinct population groups listed in Table The population element is the unit of analysis, and may be defined as an individual, household, institution, patient visit, and so on. Table Possible Population Choices for a RESEARCH Study by a Medical Practice Group Members Where When All patients Designated group practice Last 12 months Patients who have had orthopedic work Designated group practices Last 12 months All people Specified geographic area Last 12 months All people who have had orthopedic work Specified geographic area Last 12 months The second and third columns of Table define population choices in terms of its location and timeframe.

8 One useful approach is to first define the ideal population to meet study objectives, and then apply practical constraints to ultimately limit and define the study population. However over defining the population should be avoided unless it is absolutely necessary. Over defining can limit the extent to which findings can be generalized, and greatly increase the operational cost and difficulty of finding population elements (Sudman, 1976). 125 Scott M. Smith and Gerald S. Albaum, An Introduction to Marketing RESEARCH , 2010 Census, or Sample?

9 Once the population has been defined, the investigator must decide whether to conduct the survey among all members of the population, or only a sample subset of the population. The desirability and advantages of using a sample rather than a census depend on a variety of factors such as geographic location, the absolute size of the population, and the sample size required for results sufficiently accurate and precise to achieve the required purposes. Two major advantages of using a sample rather than a census are speed and timeliness.

10 A survey based on a sample takes much less time to complete than one based on a census. Frequently, the use of a sample results is a notable economy of time, money and effort, especially when a census requires hiring, training and supervising many people. In other situations, a sample is necessary because of the destructive nature of the measurement, such as in product testing. There is a related problem over surveying human populations when many different surveys need to be conducted on the same population within a relatively short period of time.