SECTION 1.3 Data Collection and Experimental Design 19 Experimental Design To produce meaningful unbiased results, experiments should be carefully designed and executed. It is important to know what steps should be taken to make the results of an experiment valid. Three key elements of a well-designed experiment are control, randomization, and replication. Because experimental results can be ruined by a variety of factors, being able to control these influential factors is important. One such factor is a confounding variable. A confounding variable occurs when an experimenter cannot tell the difference between the effects of different factors on the variable. DEFINITION For instance, to attract more customers, a coffee shop owner experiments by remodeling the shop using bright colors. At the same time, a shopping mall nearby has its grand opening. If business at the coffee shop increases, it cannot be determined whether it is because of the new colors or the new shopping mall. The effects of the colors and the shopping mall have been confounded. Another factor that can affect experimental results is the placebo effect. The placebo effect occurs when a subject reacts favorably to a placebo when in fact the subject has been given a fake treatment. To help control or minimize the placebo effect, a technique called blinding can be used. Blinding is a technique in which the subjects do not know whether they are receiving a treatment or a placebo. In a double-blind experiment, neither the experimenter nor the subjects know whether the subjects are receiving a treatment or a placebo. The experimenter is informed after all the data have been collected. This type of experimental design is preferred by researchers. DEFINITION One challenge for experimenters is assigning subjects to groups so the groups have similar characteristics (such as age, height, weight, and so on). When treatment and control groups are similar, experimenters can conclude that any differences between groups are due to the treatment. To form groups with similar characteristics, experimenters use randomization. Randomization is a process of randomly assigning subjects to different treatment groups. DEFINITION In a completely randomized design, subjects are assigned to different treatment groups through random selection. In some experiments, it may be necessary for the experimenter to use blocks, which are groups of subjects with similar characteristics. A commonly used experimental design is a randomized block design. To use a randomized block design, the experimenter divides the subjects with similar characteristics into blocks, and then, within each block, randomly assign subjects to treatment groups. For instance, an experimenter who is testing the effects of a new weight loss drink may first divide the subjects into age categories such as 30–39 years old, 40–49 years old, and over 50 years old, and then, within each age group, randomly assign subjects to either the treatment group or the control group (see figure at the left). Study Tip The Hawthorne effect occurs in an experiment when subjects change their behavior simply because they know they are participating in an experiment. Randomized Block Design Control Treatment 30–39 years old Control Treatment 40–49 years old Control Treatment Over 50 years old All subjects
RkJQdWJsaXNoZXIy NjM5ODQ=