# The disadvantages of within-subjects designs

1. Chuck Wagon is very excited about the within-subjects approach. "Now I'll never need to run large numbers of subjects again," he says. However Chuck has forgotten that within-subjects designs may be a) useless, b) impossible, c) confounded by order effects, or d) impractical when excessive subject time spent in an experiment makes data inaccurate. Give an example of each of these four objections.

2. For each of the following examples, explain whether the researcher has made a correct decision or has made a Type 1 or Type 2 error. Explain why. a) Dr. G rejects the null hypothesis although the independent variable had no effect. b) Dr. R rejects the null hypothesis when it is false. c) Although the independent variable had an effect, Dr. E does not reject the null hypothesis.

3. A researcher has studied subjects' ability to learn to translate words into Morse code. He has experimented with two treatment conditions: in one condition, the subjects are given massed practice; they spend 8 full hours on the task. In the other condition, subjects are given distributed practice; they also spend 8 hours, but their practice is spread over four days, practicing 2 hours at a time. After the practice, all subjects are given a test message to encode; the dependent variable is the number of errors made. The researcher has matched the subjects on intelligence. The results are in the following table. Decide which statistical test would be appropriate, carry out the test, and evaluate the outcome. Assume a significance level of .05 and that the direction of the outcome has not been predicted. Massed Practice Distributed Practice S1 6 S1 5 S2 4 S2 3 S3 3 S3 2 S4 5 S4 2 S5 2 S5 3

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#### Solution Preview

1. Example of "useless": When you conduct within-subjects design, two groups are dependent. But in some cases, groups are independent. So the design is valid.

Example of "impossible": When you conduct within-subjects design, you fail to find a sample which can represent the population.

Example of "confounded by ...

#### Solution Summary

The solution first discusses the disadvantages of within-subjects designs and then perform the matched pair t test.