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CHAPTER 7: Repeated Measures Designs 229Defining Practice EffectsThe repeated measures designs have another important advantage in additionto the ones we have already described. In a repeated measures design, thecharacteristics of the participants cannot confound the independent variable beingmanipulated in the experiment. The same participants are tested in all the conditionsof a repeated measures design, so it is impossible to end up with brighter,healthier, or more motivated participants in one condition than in another condition.Stated more formally, there can be no confounding by individual differencesvariables in repeated measures designs. The absence of the potential for confoundingby individual differences variables is a great advantage of the repeated measuresdesigns. This does not mean, however, that there are no threats to the internalvalidity of experiments that are done using repeated measures designs.One potential threat to internal validity arises because participants maychange over time. The repeated testing of participants in the repeated measuresdesign gives them practice with the experimental task. As a result of thispractice, participants may get better and better at doing the task because theylearn more about the task, or they may get worse at the task because of such factorsas fatigue and boredom (see Figure 7.1). The changes participants undergoFIGURE 7.1There are both positive and negative effects of practicing a new skill. Repeating the sameexperience can lead to improvement, but it also can lead to fatigue, a decrease in motivation,and even boredom.