Each of the following research designs has a potential threat to the internal validity of the research. For each, indicate what the confounding variable is and how it might have been eliminated.

a. The Pepsi-Cola Company conducted the "Pepsi Challenge" by randomly assigning individuals to taste either a Pepsi or a Coke. The researchers labeled the glasses with only an "M" (Pepsi) or a "Q" (Coke) and asked the participants to indicate which they preferred. The research showed that subjects overwhelmingly preferred glass "M" over glass "Q." Why can't the researchers conclude that Pepsi was preferred to Coke?

b. Researchers gave White college students two résumés in an experiment in which they were asked to play the role of an employment officer. The résumés were designed to have equal qualifications, but one had a photo of an African American applicant attached, and the other had a photo of a White applicant. The researcher found that there were no significant differences between the evaluations of the Black applicant and the White applicant. Why can't the researcher conclude that the student's judgments were not influenced by the race of the applicant?

c. In a study of helping behavior, Ellsworth and Langer (1976) predicted that when the person who needed help made eye contact with the potential helper, situations in which the need for help was clear and unambiguous would produce more helping than would situations in which the need for help was less clear. To manipulate the ambiguity of the need for help, participants were randomly assigned to discover a person who had lost a contact lens, whereas in the other condition the person in need of help was apparently ill. Even if more help was given in the latter condition than the former, why should the researchers not conclude that it is the ambiguity of the situation that caused the difference?

d. McCann and Holmes (1984) tested the hypothesis that exercise reduces depression. They randomly assigned depressed undergraduate women either to an exercise condition (attending an aerobics class a couple of times a week for ten weeks) or to a relaxation training condition (the individuals relaxed at home by watching a videotape over the same period of time). Although the results showed that the exercise group reported less depression at the end of the ten-week period than did the relaxation group, why can't the researchers conclude that exercise reduces depression?

e. Ekman, Friesen, and Scherer (1976) tested whether lying influenced one's voice quality. Participants were randomly assigned to view either a pleasant fi lm or an unpleasant fi lm, but all of the participants were asked to describe the fi lm they saw as being pleasant. (Thus, the subjects who watched the unpleasant fi lm had to lie about what they saw.) An analysis of voice quality showed that participants used significantly higher voices when they were describing the unpleasant fi lm rather than the pleasant fi lm. Why can't the authors conclude that lying produced the differences in voice quality?

f. A researcher studying the "mere exposure" phenomenon (Zajonc, 1980) wants to show that people like things more if they have seen them more often. He shows a group of participants a list of twenty words at an experimental session. One week later, the participants return for a second session in which they are randomly assigned to view either the same words again or a different set of twenty words, before indicating how much they like the twenty words that everyone had seen during the fi rst session. The results show that the participants who have now seen the words twice like the words better than the group that only saw the words once. Why can't the researcher conclude that people like the words more because they have seen them more often?

g. A researcher wants to show that people with soft voices are more persuasive than people with harsh voices. She has a male actor with a loud voice give an appeal to one set of participants and a woman with a soft voice give the exact same appeal to another set of participants. The researcher finds that the soft voice is indeed more persuasive because people change their attitudes more after hearing the appeal from the female. Why can't the researcher conclude that soft voices are more persuasive?

h. An elementary school teacher wants to show that parents' involvement helps their children learn. She randomly chooses one half of the boys and one half of the girls in her class and sends a note home with them. The note asks the parents to spend more time each day working with the child on his or her math homework. The other half of the children do not receive a note. At the end of the school year, the teacher finds that the children whose parents she sent notes to have significantly better final math grades. Why can't the researcher conclude that parental involvement increased the students' scores?

i. Employees in a large factory are studied to determine the influence of providing incentives on task performance. Two similar assembly rooms are chosen for the study. In one room, the experimenters talk about the research project that is being conducted and explain that the employees will receive a reward for increased performance: Each worker will receive a weekly bonus if he or she increases his or her performance by 10 percent. In the other room, no mention is made of any research. If the reward is found to increase the performance in the first assembly room, why can't the researchers conclude that it was the financial bonus that increased production?