Does aspirin reduce the risk of heart attacks? Is one brand of weight loss drug more effective than another? Is fatigue as dangerous to a driver as the influence of alcohol? Questions like these are answered using randomized experiments. In this module, you will learn important aspects of experimental design. Proper study design ensures the production of reliable, accurate data.
The purpose of an experiment is to investigate the relationship between two variables. When one variable causes change in another, we call the first variable the independent variable or explanatory variable. The affected variable is called the dependent variable or response variable: stimulus, response. In a randomized experiment, the researcher manipulates values of the explanatory variable and measures the resulting changes in the response variable. The different values of the explanatory variable are called treatments. An experimental unit is a single object or individual to be measured.
You want to investigate the effectiveness of vitamin E in preventing disease. You recruit a group of subjects and ask them if they regularly take vitamin E. You notice that the subjects who take vitamin E exhibit better health on average than those who do not. Does this prove that vitamin E is effective in disease prevention? It does not. There are many differences between the two groups compared in addition to vitamin E consumption. People who take vitamin E regularly often take other steps to improve their health: exercise, diet, other vitamin supplements, choosing not to smoke. Any one of these factors could be influencing health. As described, this study does not prove that vitamin E is the key to disease prevention.
Additional variables that can cloud a study are called lurking variables. In order to prove that the explanatory variable is causing a change in the response variable, it is necessary to isolate the explanatory variable. The researcher must design her experiment in such a way that there is only one difference between groups being compared: the planned treatments. This is accomplished by the random assignment of experimental units to treatment groups. When subjects are assigned treatments randomly, all of the potential lurking variables are spread equally among the groups. At this point, the only difference between groups is the one imposed by the researcher. Different outcomes measured in the response variable, therefore, must be a direct result of the different treatments. In this way, an experiment can prove a cause-and-effect connection between the explanatory and response variables.
The power of suggestion can have an important influence on the outcome of an experiment. Studies have shown that the expectation of the study participant can be as important as the actual medication. In one study of performance-enhancing drugs, researchers noted:
Results showed that believing one had taken the substance resulted in [performance] times almost as fast as those associated with consuming the drug itself. In contrast, taking the drug without knowledge yielded no significant performance increment. (McClung, M. Collins, D. “Because I know it will!”: placebo effects of an ergogenic aid on athletic performance. Journal of Sport & Exercise Psychology. 2007 Jun. 29(3):382-94. Web. April 30, 2013.)
When participation in a study prompts a physical response from a participant, it is difficult to isolate the effects of the explanatory variable. To counter the power of suggestion, researchers set aside one treatment group as a control group. This group is given a placebo treatment–an active treatment that cannot directly influence the response variable. The control group helps researchers balance the effects of being in an experiment with the effects of the active treatments. Of course, if you are participating in a study and you know that you are receiving a pill which contains no actual medication, then the power of suggestion is no longer a factor. Blinding or masking in a randomized experiment preserves the power of suggestion. When a person involved in a research study is blinded, they don't know who is receiving the active treatment(s) and who is receiving the placebo treatment. A double-blind experiment is one in which both the subjects and the researchers involved with the subjects are blinded.
A study is done to investigate whether ingesting caffeine can affect a person's muscular endurance. Subjects performed the maximum number of repetitions (reps) on a bench press at 60% of their body weight multiple times. They completed the exercise three times after consuming a placebo (water), and three times after consuming a caffeine-laced beverage (5mg/kg body weight). Participants were assigned at random to consume the caffeine first (during the first three trials) or the placebo first (during the last three trials). For each trial, researchers recorded the total number of successful repetitions and the subject's impression of the beverage: positive, negative, or neutral.
- Describe the explanatory and response variables in this study.
- What are the treatments?
- Identify any lurking variables that could interfere with this study.
- Is it possible to use blinding in this study?
- Answer
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The explanatory variable in this study is the type of beverage consumed (Caffeine vs. Placebo), as this is the factor being manipulated or changed by the researchers. The response variable is the total number of successful repetitions completed on the bench press, as this is the outcome being measured. There are two treatments: the caffeine-laced beverage and the placebo (unscented water), with all subjects experiencing both in a crossover design. The random assignment of the treatment order eliminates potential lurking variables such as differences in baseline strength or fatigue over time. However, a major lurking variable that could interfere is the subject's typical daily caffeine consumption, which might affect their physiological response to the dose. Blinding is possible, though not for the researchers preparing the drinks; subjects can be single-blinded if the placebo and caffeine drink are indistinguishable in taste and appearance, ensuring they do not know which treatment they received.