Section 1.1 Sampling 5 Objective 2 Determine when samples of convenience are acceptable Samples of Convenience In some cases, it is difficult or impossible to draw a sample in a truly random way. In these cases, the best one can do is to sample items by some convenient method.Asample obtained in such a way is called a sample of convenience. DEFINITION A sample of convenience is a sample that is not drawn by a well-defined random method. EXAMPLE 1.6 Drawing a sample of convenience Aconstruction engineer has just received a shipment of 1000 concrete blocks, each weighing approximately 50 pounds. The blocks have been delivered in a large pile. The engineer wishes to investigate the crushing strength of the blocks by measuring the strengths in a sample of 10 blocks. Explain why it might be difficult to draw a simple random sample of blocks. Describe how the engineer might draw a sample of convenience. Solution To drawa simple random samplewould require removing blocks from the center and bottom of the pile, which might be quite difficult. One way to draw a sample of convenience would be to simply take 10 blocks off the top of the pile. CAUTION Don’t use a sample of convenience when it is possible to draw a simple random sample. Problems with samples of convenience The big problem with samples of convenience is that they may differ systematically in some way from the population. For this reason, samples of convenience should not be used, except in some situations where it is not feasible to draw a random sample. When it is necessary to draw a sample of convenience, it is important to think carefully about all the ways in which the sample might differ systematically from the population. If it is reasonable to believe that no important systematic difference exists, then it may be acceptable to treat the sample of convenience as if it were a simple random sample.With regard to the concrete blocks, if the engineer is confident that the blocks on the top of the pile do not differ systematically in any important way from the rest, then he may treat the sample of convenience as a simple random sample. If, however, it is possible that blocks in different parts of the pile may have been made from different batches of mix, or may have different curing times or temperatures, a sample of convenience could give misleading results. SUMMARY • A sample of convenience may be acceptable when it is reasonable to believe that there is no systematic difference between the sample and the population. • A sample of convenience is not acceptable when it is possible that there is a systematic difference between the sample and the population. Objective 3 Describe stratified sampling, cluster sampling, systematic sampling, and voluntary response sampling Some Other Sampling Methods Stratified sampling In stratified sampling, the population is divided into groups, called strata, where the members of each stratum are similar in some way. Then a simple random sample is drawn from each stratum. Stratified sampling is useful when the strata differ from one another, but the individuals within a stratum tend to be alike.
navidi_monk_essential_statistics_1e_ch1_3
To see the actual publication please follow the link above