28 Chapter 1 Basic Ideas The words “heavy” and “ burden” suggest that taxes are too high, and encourage a “Yes” response. A better way to ask this question is to present it as a multiple choice: “What is your opinion on decreasing taxes for middle-class families? Choices: Strongly disagree,Somewhat disagree, Neither agree nor disagree, Somewhat agree, Strongly agree.” Nonresponse bias People cannot be forced to answer questions or to participate in a study. In any study, a certain proportion of people who are asked to participate refuse to do so. These people are called nonresponders. In many cases, the opinions of nonresponders tend to differ from the opinions of those who do respond. As a result, surveys with many nonresponders are often biased. Sampling bias Sampling bias occurs when some members of the population are more likely to be included in the sample than others. For example, samples of convenience almost always have sampling bias, because people who are easy to sample are more likely to be included. It is almost impossible to avoid sampling bias completely, but modern survey organizations work hard to keep it at a minimum. A Big Sample Size Doesn’t Make Up for Bias A sample is useful only if it is drawn by a method that is likely to represent the population well. If you use a biased method to draw a sample, then drawing a big sample doesn’t help; a big nonrepresentative sample does not describe a population any better than a small nonrepresentative sample. In particular, voluntary response surveys often drawseveral hundred thousand people to participate. Although the sample is large, it is unlikely to represent the population well, so the results are meaningless. Check Your Understanding 1. Eighty thousand people attending a professional football game filled out surveys asking their opinions on using tax money to upgrade the football stadium. Seventy percent said that they supported the use of tax money. Then a pollster surveyed a simple random sample of 500 voters, and only 30% of the voters in this sample supported the use of tax money. The owner of the football team claims that the survey done at the football stadium is more reliable, because the sample size was much larger. Is he right? Explain. 2. A polling organization placed telephone calls to 1000 people in a certain city to ask them whether they favor a tax increase to build a new school. Two hundred people answered the phone, and 150 of them opposed the tax. Can you conclude that a majority of people in the city oppose the tax, or is it likely that this result is due to bias? Explain. Answers are on page 29. SECTION 1.4 Exercises Exercises 1 and 2 are the Check Your Understanding exercises located within the section. Understanding the Concepts In Exercises 3–5, fill in each blank with the appropriate word or phrase. 3. are highly unreliable in part because people who have strong opinions are more likely to participate. 4. People who are asked to participate in a study but refuse to do so are called . 5. A large sample is useful only if it is drawn by a method that is likely to represent the well. In Exercises 6–8, determine whether the statement is true or false. If the statement is false, rewrite it as a true statement. 6. The way that a question in a survey is worded rarely has an effect on the responses.
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