Repeated Measures Design

One of the most frequently used experimental designs in the health sciences field is the repeated measures design.
Definition
                 ''A repeated meausres design is one in wich measurements of the same variable are made on each subject on two or more different occasions.''

The usual motivation for using a repeated measures design is a desire to control for variability among subjects. In such a design each subject serves as its own control. When measurements are taken on only two occasions we have the paired comparisions design that we discussed. One of the most frequently encountered situations in which the repeated measures design is used is the situation in which the investigator is concerned with reponses over time.

Advantage

The major advantage of the repeated measures design is, as previously mentiond, its ability to cintrol for extraneous variation among subjects. An additional advantage is the fact that fewer subjects are needed for the repeated measures design than for a design in which different subjects are used for each occasion on which measurements are made. Suppose, for example , that we have four treatments ( in the usual sense) or four points in time on each of which we would like to have 10 measurements. If a different sample of subjects is used for each of the four treatments or points in time, 40 subjects would be required . If we are able to take measurements on the same subject for each treatment or point in time, i.e if we can use a repeated measures design, only 10 subjects would be required. This can be a very attractive advantage if subjects are scarce or expensive to recruit.

Disadvantage

A major potential problem to be on the alert for is what is known as the carry-over effect. When two or more treatments are being evaluated, the investigator should make sure that a subject's response to one treatment does not reflect a residual effect from previous treatments. This problem can frequently be solved by allowing a sufficient length of time between treatments. Another possible problem is the position effect. A subject's response to a treatment experienced last in a sequence may be different from the response that would have occured if the treatment had been first in the sequence. In certain studies, such as those involving physical participation on the part of the subjects, enthusiasm that is high at the beginning of the study may give way to boredom toward the end. A way around this problem is to randomized the sequence of treatments independently for each subject.