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sampling all listed residential telephone numbers in the United States. Of 45,956 calls to these numbers, $5029$ were completed. The goal of the survey was to estimate how far people drive, on average, per day.$14$

(a) What was the rate of nonresponse for this sample?

(b) Explain how nonresponse can lead to bias in this survey. Be sure to give the direction of the bias.

A survey paid for by makers of disposable diapers found that $84\%$ of the sample opposed banning disposable diapers. Here is the actual question:

It is estimated that disposable diapers account for less than $2\%$of the trash in today鈥檚 landfills. In contrast, beverage containers, third-class mail, and yard wastes are estimated to account for about $21\%$ of the trash in landfills. Given this, in your opinion, would it be fair to ban disposable diapers? Explain how the wording of the question could result in bias. Be sure to specify the direction of the bias.

In an interesting experiment, researchers examined the effect of ultrasound on birth weight. Pregnant women participating in the study were randomly assigned to one of two groups. The first group of women received an ultrasound; the second group did not. When the subjects鈥� babies were born, their birth weights were recorded. The women who received the ultrasounds had heavier babies.

Was the experiment double-blind? Why is this important?

Ring-no-answer A common form of nonresponse in telephone surveys is 鈥渞ing-no-answer.鈥� That is, a call is made to an active number but no one answers. The Italian National Statistical Institute looked at nonresponse to a government survey of households in Italy during the periods January $1$ to Easter and July $1$ to August $31$ All calls were made between$7$ and $10$ p.m., but $21.4\%$ gave 鈥渞ing-no-answer鈥� in one period versus $41.5\%$ 鈥渞ing-no-answer鈥� in the other period$15$ Which period do you think had the higher rate of no answers? Why? Explain why a high rate of nonresponse makes sample results less reliable.

In an interesting experiment, researchers examined the effect of ultrasound on birth weight. Pregnant women participating in the study were randomly assigned to one of two groups. The first group of women received an ultrasound; the second group did not. When the subjects鈥� babies were born, their birth weights were recorded. The women who received the ultrasounds had heavier babies.

Based on your answers to Questions $1$ and $2$, describe an improved design for this experiment

Running red lights The sample described in Exercise $31$produced a list of $5024$licensed drivers. The investigators then chose an SRS of $880$ of these drivers to answer questions about their driving habits. One question asked was: 鈥淩ecalling the last ten traffic lights you drove through, how many of them were red when you entered the intersections?鈥� Of the $880$ respondents, $171$admitted that at least one light had been red. A practical problem with this survey is that people may not give truthful answers. What is the likely direction of the bias: do you think more or fewer than $171$ of the $880$ respondents really ran a red light? Why?

Seat belt uses A study in El Paso, Texas, looked at seat belt use by drivers. Drivers were observed at randomly chosen convenience stores. After they left their cars, they were invited to answer questions that included questions about seat belt use. In all, 75% said they always used seat belts, yet only 61.5% were wearing seat belts when they pulled into the store parking lots.16 Explain the reason for the bias observed in responses to the survey. Do you expect bias in the same direction in most surveys about seat belt use?

Wording bias Comment on each of the following as a potential sample survey question. Is the question clear? Is it slanted toward the desired response?

(a) 鈥淪ome cell phone users have developed brain cancer. Should all cell phones come with a warning label explaining the danger of using cell phones?鈥�

(b) 鈥淒o you agree that a national system of health insurance should be favored because it would provide health insurance for everyone and would reduce

administrative costs?鈥�

(c) 鈥淚n view of escalating environmental degradation and incipient resource depletion, would you favor economic incentives for recycling of resource-intensive consumer goods?鈥�

Checking for bias Comment on each of the following as a potential sample survey question. Is the question clear? Is it slanted toward the desired response?

(a) Which of the following best represents your opinion on gun control?

1. The government should confiscate our guns.

2. We have the right to keep and bear arms.

(b) A freeze on nuclear weapons should be favored because it would begin a much-needed process to stop everyone in the world from building nuclear

weapons now and reduce the possibility of nuclear war in the future. Do you agree or disagree?

The Web portal AOL places opinion poll questions next to many of its news stories. Simply click your response to join the sample. One of the questions

in January $2008$ was 鈥淒o you plan to diet this year?鈥� More than $30,000$ people responded, with $68\%$ saying 鈥淵es.鈥� You can conclude that

(a) about $68\%$ of Americans planned to diet in $2008$

(b) the poll used a convenience sample, so the results tell us little about the population of all adults.

(c) the poll uses voluntary response, so the results tell us little about the population of all adults.

(d) the sample is too small to draw any conclusion.

(e) None of these.