Experimental Groups versus Control Groups
In an observational study, the researcher simply records what she finds in the world. In a scientific experiment, in contrast, the researcher deliberately changes something. She might change the nature of the test being given, or the circumstances, or the instruc-tions. This change is usually referred to as the experimental manipulation—and the point of an experiment is to ask what results from this change. To see how this plays out, let’s consider a new example.
Many companies sell audio recordings that contain subliminal messages embedded in background music. The message might be an instruction to give up smoking or curb overeating, or it might be designed to build self-esteem or overcome shyness. The mes-sage is played so softly that you can’t consciously detect it when listening to the record-ing; still, it’s alleged to provide important benefits.
Anecdotal evidence—reports from various people announcing, “Hey, I tried the tapes, and they really worked for me!”—sometimes suggests that these subliminal mes-sages can be quite effective. However, we’ve already discussed the problems with relying on such anecdotes; and so, if we want a persuasive test of these messages, it would be best to set up an experiment. Our experimental manipulation would be the presenta-tion of the subliminal message, and this would define our study’s independent variable: message presented versus message not presented.
What about the dependent variable? Suppose we’re testing a tape advertised as helping people give up cigarette smoking. In that case, our dependent variable might be the num-ber of cigarettes smoked in, say, the 24-hour period after hearing the tape. In our study, we might ask 20 students—all longtime smokers—to listen to the tape; then we’d count up how many cigarettes they each consume in the next 24 hours. However, this procedure by itself tells us nothing. If the students smoke an average of 18 cigarettes in the 24-hour test period, is that less than they would have smoked without the tape? We have no way to tell from the procedure described so far, and so there’s no way to interpret the result.
What’s missing is a basis for comparison. One way to arrange for this is to use two groups of participants. The experimental group will experience the experimental manipulation—their tape contains the subliminal message. The control group will not experience the manipulation. So, by comparing the control group’s cigarette consump-tion to that of the experimental group, we can assess the message’s effectiveness.
But exactly what procedure should we use for the control group? One possibility is for these participants to hear no recording at all, while those in the experimental group hear the tape containing the subliminal message embedded in music. This setup, how-ever, once again creates problems: If we detect a contrast between the two groups, then the subliminal message might be having the predicted effect. But, on the other hand, notice that the subliminal message is embedded in music—so is the experimental group being influenced by the music rather than the message? (Perhaps the partici-pants find it relaxing to listen to music and then smoke less because they’re more relaxed.) In this case, it helps to listen to the recording; but the result would be the same if there had been no subliminal message at all.
To avoid this ambiguity, the procedures used for the control group and the experi-mental group must match in every way except for the experimental manipulation. If the experimental group hears music containing the subliminal message, the control group must hear the identical music without any subliminal message. If the procedure for the experimental group requires roughly 30 minutes, then the procedure for the control participants should take 30 minutes. It’s also important for the investigators to treat the two groups in precisely the same way. If we tell members of the experimental group they’re participating in an activity that might help them smoke less, then we must tell members of the control group the same thing. That way, the two groups will have sim-ilar expectations about the procedure.
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