Review Exercises 31 11. In a study, 200 patients with skin cancer are randomly divided into two groups. One group receives an experimental drug and the other group receives a placebo. Is this a randomized experiment or an observational study? 12. In a randomized experiment, if there are large differences in outcomes among treatment groups, we can conclude that the differences are due to the . 13. In analyzing the course grades of students in an elementary statistics course, a professor notices that students who are seniors performed better than students who are sophomores. The professor is tempted to conclude that older students perform better than younger ones. Describe a possible confounder in this situation. 14. True or false: The way that questions are worded on a survey may have an effect on the responses. 15. A radio talk show host invites listeners to call the show to express their opinions about a political issue. How reliable is this survey? Explain. Review Exercises 1. Qualitative or quantitative? Is the number of points scored in a football game qualitative or quantitative? 2. Nominal or ordinal? Is the color of an MP3 player nominal or ordinal? 3. Discrete or continuous? Is the area of a college campus discrete or continuous? 4. Which type of variable is it? A theater concession stand sells soft drink and popcorn combos that come in sizes small, medium, large, and jumbo. True or false: a. Size is a qualitative variable. b. Size is an ordinal variable. c. Size is a continuous variable. In Exercises 5–8, identify the kind of sample that is described. 5. Website ratings: A popular website is interested in conducting a survey of 400 visitors to the site in such a way that 200 of them will be under age 30, 150 will be aged 30–55, and 50 will be over 55. 6. Favorite performer: Viewers of a television show are asked to vote for their favorite performer by sending a text message to the show. 7. School days: A researcher selects 4 of 12 high schools in a certain region and surveys all of the administrative staff members in each school about a potential change in the ordering of supplies. Which type of sample does this represent? 8. Political polling: A pollster obtains a list of registered voters and uses a computer random number generator to choose 100 of them to ask which candidate they prefer in an upcoming election. 9. Fluoride and tooth decay: Researchers examine the association between the fluoridation of water and the prevention of tooth decay by comparing the prevalence of tooth decay in countries that have fluoridated water with the prevalence in countries that do not. a. Is this a randomized experiment or an observational study? b. Assume that tooth decay was seen to be less common in countries with fluoridated water. Could this result be due to confounding? Explain. 10. Better gas mileage: A taxi company in a large city put a new type of tire with a special tread on a random sample of 50 cars, and the regular type of tire on another random sample of 50 cars. After a month, the gas mileage of each car was measured. a. Is this a randomized experiment or an observational study? b. Assume that one of the samples had noticeably better gas mileage than the other. Could this result be due to confounding? Explain. 11. Cell phones and driving: To determine whether using a cell phone while driving increases the risk of an accident, a researcher examines accident reports to obtain data about the number of accidents in which a driver was talking on a cell phone. a. Is this a randomized experiment or an observational study? b. Assume that the accident reports show that people were more likely to have an accident while talking on a cell phone. Could this result be due to confounding? Explain. 12. Turn in your homework: The English department at a local college is considering using electronic-based assignment submission in its English composition classes. To study its effects, each section of the class is divided into two groups at random. In one group, assignments are submitted by turning them in to the professor on paper. In the other group, assignments are submitted electronically. a. Is this a randomized experiment or an observational study? b. Assume that the electronically submitted assignments had many fewer typographical errors, on average, than the ones submitted on paper. Could this result be due to confounding? Explain. In Exercises 13–15, explain why the results of the studies described are unreliable. 13. Which TV station do you watch? The TV columnist for a local newspaper invites readers to log on to a website to vote for their favorite TV newscaster.
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