20 Chapter 1 Basic Ideas The raltegravir study was a randomized experiment, because the treatments were assigned to the patients at random. What are the advantages of randomized experiments? In a perfect study, the treatment groups would not differ from each other in any important way except that they receive different treatments. Then, if the outcomes differ among the groups, we may be confident that the differences in outcome must have been caused by differences in treatment. In practice, it is impossible to construct treatment groups that are exactly alike. But randomization does the next best thing. In a randomized experiment, any differences between the groups are likely to be small. In addition, the differences are due only to chance. Because the raltegravir studywas a randomized experiment, it is reasonable to conclude that the higher success rate in the raltegravir group was actually due to raltegravir. SUMMARY In a randomized experiment, if there are large differences in outcomes among the treatment groups, we can conclude that the differences are due to the treatments. EXAMPLE 1.14 Identifying a randomized experiment To assess the effectiveness of a new method for teaching arithmetic to elementary school children, a simple random sample of 30 first graders was taught with the new method, and another simple random sample of 30 first graderswas taught with the currently used method. At the end of eight weeks, the children were given a test to assess their knowledge. What are the treatments in this study? Explain why this is a randomized experiment. Solution The treatments are the two methods of teaching. This is a randomized experiment because children were assigned to the treatment groups at random. Double-blind experiments We have described the advantages of assigning treatments at random. It is a further advantage if the assignment can be done in such a way that neither the experimenters nor the subjects know which treatment has been assigned to which subject. Experiments like this are called double-blind experiments. The raltegravir experiment was a double-blind experiment, because neither the patients nor the doctors treating them knew which patients were receiving the drug and which were receiving the placebo. DEFINITION An experiment is double-blind if neither the investigators nor the subjects know who has been assigned to which treatment. Experiments should be run double-blind whenever possible, because when investigators or subjects know which treatment is being given, they may tend to report the results differently. For example, in an experiment to test the effectiveness of a new pain reliever, patients who know they are getting the drug may report their pain levels differently than those who know they are taking a placebo. Doctors can be affected as well; a doctor’s diagnosis may be influenced by a knowledge of which treatment a patient received. In some situations, it is not possible to run a double-blind experiment. For example, in an experiment that compares a treatment that involves taking medication to a treatment that involves surgery, both patients and doctors will know who got which treatment. EXAMPLE 1.15 Determining whether an experiment is double-blind Is the experiment described in Example 1.14 a double-blind experiment? Explain.
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