764 CHAPTER 12 Statistics Convenience Sampling A convenience sample uses data that are easily or readily obtained. Occasionally, data that are conveniently obtained may be all that is available. In some cases, some information is better than no information at all. Nevertheless, convenience sampling can be extremely biased. For example, suppose that a town wants to raise taxes to build a new elementary school. The local newspaper wants to obtain the opinion of some of the residents and sends a reporter to a senior citizens center. The first 10 people who exit the building are asked if they are in favor of raising taxes to build a new school. This sample could be biased against raising taxes for the new school. Most senior citizens would not have school-age children and may not be interested in paying increased taxes to build a new school. Although a convenience sample may be very easy to select, one must be very cautious when using the results obtained by this method. A form of convenience sample is the voluntary response sample . A voluntary response sample is a sample in which participants voluntarily choose to participate as a part of the sample. For example, a company asks for volunteers to give product feedback through an Internet survey. The only participants in this sample are those who volunteer to respond. Participants in a voluntary response sample usually respond because they have a strong opinion on the subject of the survey. As with any convenience sample, one must be cautious when using results obtained from a volunteer response sample. Did You Know? Don’t Count Your Votes Until They’re Cast A classic instance of faulty sampling occurred in the 1936 U.S. presidential election. On the basis of the responses of 2,300,000 voters, selected from automobile owners and telephone subscribers, the Literary Digest confidently predicted that the Republican candidate, Alf Landon, would be elected. As it turned out, Franklin D. Roosevelt, the Democratic candidate, won by a large margin. The erroneous prediction occurred because the voters used in the sample were not representative of the general voting population. In 1936, telephones and automobiles were unaffordable to the average voter. Example 1 Identifying Sampling Techniques Identify the sampling technique used to obtain a sample in the following. Explain your answer. a) Every 20th smartwatch coming off an assembly line is checked for defects. b) A $50 gift certificate is given away at the National Nurses Convention. Tickets are placed in a bin, and the tickets are mixed up. Then the winning ticket is selected by a blindfolded person. c) Children in a large city are classified based on the neighborhood school they attend. A random sample of five schools is selected. All the children from each selected school are included in the sample. d) The first 50 people entering a zoo are asked if they support an increase in taxes to support a zoo expansion. e) All students at St. Petersburg College are classified according to their program of study. A random sample of students from each program of study is selected. Solution a) Systematic sampling. The sample is obtained by drawing every n th item. In this example, every 20th item on an assembly line is selected. b) Random sampling. Every ticket has an equal chance of being selected. c) Cluster sampling. A random sample of geographic areas is selected. d) Convenience sampling. The sample is selected by picking data that are easily obtained. e) Stratified sampling. The students are divided into strata based on their program of study. Then random samples are selected from each strata. 7 Now try Exercise 11 Learning Catalytics Keyword: Angel-SOM-12.1 (See Preface for additional information.) Misuses of Statistics Statistics, when used properly, is a valuable tool to society. However, many individuals, businesses, and advertising firms misuse statistics to their own advantage. You should examine statistical statements very carefully before accepting them as fact. You should ask yourself two questions: Was the sample used to gather the statistical data unbiased and of sufficient size? Is the statistical statement ambiguous; that is, can it be interpreted in more than one way? Michelle D. Milliman/Shutterstock
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