<p style="text-align:justify"><span style="font-size:14px">Does aspirin reduce the risk of heart attacks? Is one brand of fertilizer more effective at growing roses than another? Is fatigue as dangerous to a driver as speeding? 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.</span></p>

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<p style="text-align:justify"><span style="font-size:14px">The purpose of an experiment is to investigate the relationship between two variables. In an experiment, there is the <strong>explanatory variable </strong>which affects the <strong>response variable</strong>. In a randomized experiment, the researcher manipulates the explanatory variable and then observes the response variable. Each value of the explanatory variable used in an experiment is called a <strong>treatment</strong>.</span></p>

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<p style="text-align:justify"><span style="font-size:14px">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. 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.</span></p>

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<p style="text-align:justify"><span style="font-size:14px">Additional variables that can cloud a study are called <strong>lurking variables</strong>. 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 <strong>random assignment </strong>of experimental units to treatment groups. When subjects are assigned treatments <span style="color:#424242">called&nbsp;<strong>treatments</strong>. An&nbsp;<strong>experimental unit</strong>&nbsp;is a single object or individual to be measured.</span></span></p>

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<p style="text-align:justify"><span style="font-size:14px"><span style="background-color:white"><span style="color:black">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:</span></span></span></p>

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<p style="text-align:justify"><span style="font-size:14px">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.</span></p>

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<p style="text-align:justify"><span style="font-size:14px">Confounding occurs when the effects of multiple factors on a response cannot be separated, for instance, if a student guesses on the even-numbered questions on an exam and sits in a favorite spot on exam day. Why does the student get a high test scores on the exam? It could be the increased study time or sitting in the favorite spot or both. Confounding makes it difficult to draw valid conclusions about the effect of each factor on the outcome. The way around this is to test several outcomes with one method (treatment). This way, we know which treatment really works. 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 substances, researchers noted the following:</span></p>

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<p style="text-align:justify"><span style="font-size:14px">Results showed that believing one had taken the substance resulted in [performance] times almost as fast as those associated with consuming the substance itself. In contrast, taking the substance without knowledge yielded no significant performance increment. </span></p>

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<p style="text-align:justify"><span style="font-size:14px">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 <strong>control group</strong>.</span></p>

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<p style="text-align:justify"><span style="font-size:14px">This group is given a <strong>placebo </strong>treatment, a treatment that cannot 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 that contains no actual medication, then the power of suggestion is no longer a factor. <strong>Blinding </strong>in a randomized experiment designed to reduce bias by hiding information. When a person involved in a research study is blinded, he does not know who is receiving the active treatment(s) and who is receiving the placebo treatment. A <strong>double-blind experiment </strong>is one in which both the subjects and the researchers involved with the subjects are blinded.</span></p>

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<p style="text-align:justify"><span style="font-size:14px">Sometimes, it is neither possible nor ethical for researchers to conduct experimental studies. For example, if you want to investigate whether malnutrition affects elementary school performance in children, it would not be appropriate to assign an experimental group to be malnourished. In these cases, <strong>observational studies </strong>or <strong>surveys </strong>may be used. In an observational study, the researcher does not directly manipulate the independent variable. Instead, he or she takes recordings and measurements of naturally occurring phenomena. By sorting these data into control and experimental conditions, the &nbsp;reacher&rsquo;s measurements consist of questionnaires that are answered by the research participants.</span></p>