Answer to Question 1
(a) There is greater efficiency in data collection because we need fewer participants and we may also need to spend less time collecting data if two or more conditions can be included in a single testing session that may not increase in length that much with the additions.
(b) You can increase the validity of the data, particularly in the case of subjective judgments by participants. With repeated measures, the same person brings the same perspective in the different conditions.
(c) If the availability of participants with particular characteristics is limited, repeated measures can be useful because you don't have to worry about going out to find more scarce people.
(d) With the same people in each condition, there will be a reduction in measurement error because each person serves as his or her own control. Thus, individual differences become less of a problem across groups because you can compare a person in condition one with his or her performance in condition two.
Answer to Question 2
When participants engage in subjective ratings, as on a Likert-type scale, each person brings his or her own perspective to the judgment task. There is no guarantee that two different people will have the same subjective rating perspective. With objective measures, we rely less on personal perspectives and instead focus on more externally verifiable behaviors.
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