Repeated Measures (Design)

In designed experiments, a repeated measures design (sometimes called a within-subject design) is one where measurements are taken repeatedly on each of the experimental units. Typically, the repeated measures are taken across time. Alternatively, a test may be performed on the same experimental unit under various conditions. The analysis of this type of design is different from standard ANOVA, as some observations are correlated with each other. One of the main advantages of repeated measures designs is their "sensitivity": They can allow for analyses based on difference-measures taken on the same experimental units, thus allowing control for error variance that is due to individual differences between the units (each observation serves as its own control over consecutive repeated measurements). Another advantage is the economy of the data collection process, because fewer experimental units are needed to be tested, compared to the between-group designs. For some research problems, the main disadvantage of repeated measures designs is that they may require controlling for carry-over effects. Also, there are many research problems that do not lend themselves to be tested using the repeated measure designs (e.g., whenever one treatment changes or destroys the experimental unit making it unsuitable for any other treatments.