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To manipulate an independent variable, you have to construct an operational definition of the variable (see Chapter 4). That is, you must turn a conceptual variable into a set of operations—specific instructions, events, and stimuli to be presented to the research participants. The manipulation of the independent variable, then, is when a researcher changes the conditions to which participants are exposed. In addition, the independent and dependent variables must be introduced within the context of the total experimental setting. This has been called setting the stage (Aronson, Brewer, & Carlsmith, 1985).

Setting the Stage

In setting the stage, you usually have to supply the participants with the information necessary for them to provide their informed consent to participate (informed consent is covered in Chapter 3). This generally includes information about the underlying rationale of the study. Sometimes, the rationale given is completely truthful, although only rarely will you want to tell participants the actual hypothesis. For example, you might say that you are conducting an experiment on memory when, in fact, you are studying a specific aspect of memory (your independent variable). If participants know what you are studying, they may try to confirm (or even deny) the hypothesis, or they may try to look good by behaving in the most socially acceptable way. If you find that deception is necessary, you have a special obligation to address the deception when you debrief the participants at the conclusion of the experiment. MANIPULATING THE INDEPENDENT VARIABLE.

There are no clear-cut rules for setting the stage, except that the experimental setting must seem plausible to the participants, nor are there any clear-cut rules for translating conceptual variables into specific operations. Exactly how the variable is manipulated depends on the variable and the cost, practicality, and ethics of the procedures being considered.

Types of Manipulations

Straightforward manipulations Researchers are usually able to manipulate an independent variable with relative simplicity by presenting written, verbal, or visual material to the participants. Such straightforward manipulations manipulate variables with instructions and stimulus presentations. Stimuli may be presented verbally, in written form, via videotape, or with a computer. Let’s look at a few examples. MANIPULATING THE INDEPENDENT VARIABLE.

Goldstein, Cialdini, and Griskevicius (2008) were interested in the influence of signs that hotels leave in their bathrooms encouraging guests to reuse their towels. In their research, they simply printed signs that were hooked on towel shelves in the rooms of single guests staying at least two nights. In a standard message, the sign read “HELP SAVE THE ENVIRONMENT. You can show your respect of nature and help save the environment by reusing Page 182towels during your stay.” In this case, 35% of the guests reused their towels on the second day. Another condition invoked a social norm that other people are reusing towels: “JOIN YOUR FELLOW GUESTS IN HELPING TO SAVE THE ENVIRONMENT. Almost 75% of guests who are asked to participate in our new resource savings program do help by using their towels more than once. You can join your fellow guests in this program to save the environment by reusing your towels during your stay.” This sign resulted in 44% reusing their towels. As you might expect, the researchers have extended this research to study ways that the sign can be even more effective in increasing conservation.

Most memory research relies on straightforward manipulations. For example, Coltheart and Langdon (1998) displayed lists of words to participants and later measured recall. The word lists differed on phonological similarity: Some lists had words that sounded similar, such as cat, map, and pat, and other lists had dissimilar words such as mop, pen, and cow. They found that lists with dissimilar words are recalled more accurately. MANIPULATING THE INDEPENDENT VARIABLE.

Educational programs are most often straightforward. Pawlenko, Safer, Wise, and Holfeld (2013) examined the effectiveness of three training programs designed to improve jurors’ ability to evaluate eyewitness testimony. Subjects viewed one of three 15-minute slide presentations on a computer screen. The Interview-Identification-Eyewitness training focused on three steps to analyze eyewitness evidence: Ask if the eyewitness interviews were done properly, ask if identification methods were proper, and evaluate if the conditions of the crime scene allowed for an accurate identification. A second presentation termed “Biggers training” was a presentation of five eyewitness factors that the Supreme Court determined should be used (developed in a case called Neil v. Biggers). The Jury Duty presentation was a summary of standard information provided to jurors such as the need to be fair and impartial and the importance of hearing all evidence before reaching a verdict. After viewing the presentations, subjects read a trial transcript that included problems with the eyewitness identification procedures. The subjects in the Interview-Identification-Eyewitness conditions were most likely to use these problems in reaching a verdict.

As a final example of a straightforward manipulation, consider a study by Mazer, Murphy, and Simonds (2009) on the effect of college teacher self-disclosure (via Facebook) on perceptions of teacher effectiveness. For this study, students read one of three Facebook profiles that were created for a volunteer teacher, one for each of the high-, medium-, and low-disclosure conditions. Level of disclosure was manipulated by changing the number and nature of photographs, biographical information, favorite movies/books/quotes, campus groups, and posts on “the wall.” After viewing the profile to which they were assigned, participants rated the teacher on several dimensions. Higher disclosure resulted in perceptions of greater caring and trustworthiness; however, disclosure was not related to perceptions of teacher competence.

You will find that most manipulations of independent variables in all areas of research are straightforward. Researchers vary the difficulty of material to Page 183be learned, motivation levels, the way questions are asked, characteristics of people to be judged, and a variety of other factors in a straightforward manner. MANIPULATING THE INDEPENDENT VARIABLE.

Staged manipulations Other manipulations are less straightforward. Sometimes, it is necessary to stage events during the experiment in order to manipulate the independent variable successfully. When this occurs, the manipulation is called a staged manipulation or event manipulation.




Staged manipulations are most frequently used for two reasons. First, the researcher may be trying to create some psychological state in the participants, such as frustration, anger, or a temporary lowering of self-esteem. For example, Zitek and her colleagues studied what is termed a sense of entitlement (Zitek, Jordan, Monin, & Leach, 2010). Their hypothesis is that the feeling of being unfairly wronged leads to a sense of entitlement and, as a result, the tendency to be more selfish with others. In their study, all participants played a computer game. The researchers programmed the game so that some participants would lose when the game crashed. This is an unfair outcome, because the participants lost for no good reason. Participants in the other condition also lost, but they thought it was because the game itself was very difficult. The participants experiencing the broken game did in fact behave more selfishly after the game; they later allocated themselves more money than deserved when competing with another participant.

Second, a staged manipulation may be necessary to simulate some situation that occurs in the real world. Recall the Milgram obedience experiment that was described in Chapter 3. In that study, an elaborate procedure—ostensibly to study learning—was constructed to actually study obedience to an authority. Or consider a study on computer multitasking conducted by Bowman, Levine, Waite, and Gendron (2010), wherein students read academic material presented on a computer screen. In one condition, the participants received and responded to instant messages while they were reading. Other participants did not receive any messages. Student performance on a test was equal in the two conditions. However, students in the instant message condition took longer to read the material (after the time spent on the message was subtracted from the total time working on the computer). MANIPULATING THE INDEPENDENT VARIABLE.

Staged manipulations frequently employ a confederate (sometimes termed an “accomplice”). Usually, the confederate appears to be another participant in an experiment but is actually part of the manipulation (we discussed the use of confederates in Chapter 3). A confederate may be useful to create a particular social situation. For example, Hermans, Herman, Larsen, and Engels (2010) studied whether food intake by males is affected by the amount of food consumed by a companion. Participants were recruited for a study on evaluation of movie trailers. The participant and a confederate sat in a comfortable setting in which they viewed and evaluated three trailers. They were then told that they needed a break before viewing the next trailers; snacks were available if they were interested. In one condition, the confederate took a large serving of snacks. A small serving was taken in another condition, and the confederate Page 184did not eat in the third condition. The researchers then measured the amount consumed by the actual participants; they did model the amount consumed by the confederate but only when they were hungry.


Example of the Asch line judgment task

The classic Asch (1956) conformity experiment provides another example of how confederates may be used. Asch gathered people into groups and asked them to respond to a line judgment task such as the one in Figure 9.1. Which of the three test lines matches the standard? Although this appears to be a simple task, Asch made it more interesting by having several confederates announce the same incorrect judgment prior to asking the actual participant; this procedure was repeated over a number of trials with different line judgments. Asch was able to demonstrate how easy it is to produce conformity—participants conformed to the unanimous majority on many of the trials even though the correct answer was clear. Finally, confederates may be used in field experiments as well as laboratory research. As described in Chapter 4, Lee, Schwarz, Taubman, and Hou (2010) studied the impact of public sneezing on the perception of unrelated risks by having an accomplice either sneeze or not sneeze (control condition) while walking by someone in a public area of a university. A researcher then approached those people with a request to complete a questionnaire, which they described as a “class project.” The questionnaire measured participants’ perceptions of average Americans’ risk of contracting a serious disease. The researchers found that, indeed, being around a person who sneezes increases self-reported perception of risk.

As you can see, staged manipulations demand a great deal of ingenuity and even some acting ability. They are used to involve the participants in an ongoing social situation that the individuals perceive not as an experiment but as a real experience. Researchers assume that the result will be natural behavior that truly reflects the feelings and intentions of the participants. However, such procedures allow for a great deal of subtle interpersonal communication that is hard to put into words; this may make it difficult for other researchers to replicate the experiment. Also, a complex manipulation is difficult to interpret. If many things happened during the experiment, what one thing was responsible for the results? In general, it is easier to interpret results when the manipulation is relatively straightforward. However, the nature of the variable you are studying sometimes demands complicated procedures. MANIPULATING THE INDEPENDENT VARIABLE.

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Strength of the Manipulation

The simplest experimental design has two levels of the independent variable. In planning the experiment, the researcher has to choose these levels. A general principle to follow is to make the manipulation as strong as possible. A strong manipulation maximizes the differences between the two groups and increases the chances that the independent variable will have a statistically significant effect on the dependent variable.

To illustrate, suppose you think that there is a positive linear relationship between attitude similarity and liking (“birds of a feather flock together”). In conducting the experiment, you could arrange for participants to encounter another person, a confederate. In one group, the confederate and the participant would share similar attitudes; in the other group, the confederate and the participant would be dissimilar. Similarity, then, is the independent variable, and liking is the dependent variable. Now you have to decide on the amount of similarity. Figure 9.2 shows the hypothesized relationship between attitude similarity and liking at 10 different levels of similarity. Level 1 represents the least amount of similarity with no common attitudes, and level 10 the greatest (all attitudes are similar). To achieve the strongest manipulation, the participants in one group would encounter a confederate of level 1 similarity; those in the other group would encounter a confederate of level 10 similarity. This would result in the greatest difference in the liking means—a 9-point difference. A weaker manipulation—using levels 4 and 7, for example—would result in a smaller mean difference.

A strong manipulation is particularly important in the early stages of research, when the researcher is most interested in demonstrating that a relationship does, in fact, exist. If the early experiments reveal a relationship between the variables, subsequent research can systematically manipulate the other levels of the independent variable to provide a more detailed picture of the relationship. MANIPULATING THE INDEPENDENT VARIABLE.


Relationship between attitude similarity and liking

Page 186The principle of using the strongest manipulation possible should be tempered by at least two considerations. The first concerns the external validity of a study: The strongest possible manipulation may entail a situation that rarely, if ever, occurs in the real world. For example, an extremely strong crowding manipulation might involve placing so many people in a room that no one could move—a manipulation that might significantly affect a variety of behaviors. However, we would not know if the results were similar to those occurring in more common, less crowded situations, such as many classrooms or offices.

A second consideration is ethics: A manipulation should be as strong as possible within the bounds of ethics. A strong manipulation of fear or anxiety, for example, might not be possible because of the potential physical and psychological harm to participants.

Cost of the Manipulation

Cost is another factor in the decision about how to manipulate the independent variable. Researchers who have limited monetary resources may not be able to afford expensive equipment, salaries for confederates, or payments to participants in long-term experiments. Also, a manipulation in which participants must be run individually requires more of the researcher’s time than a manipulation that allows running many individuals in a single setting. In this respect, a manipulation that uses straightforward presentation of written or verbal material is less costly than a complex, staged experimental manipulation. Some government and private agencies offer grants for research; because much research is costly, continued public support of these agencies is very important.

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