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Abstract

A crossover study is a repeated measures design in which each subject is randomly assigned to a sequence of treatments, including at least two treatments. The most damning characteristic of a crossover study is the potential of a carryover effect of one treatment to the next period. To solve the first-order crossover problem characteristic in the classic AB|BA design, the design must be extended. One alternative uses additional treatment sequences in two periods; a second option is to add a third period and repeat one of the treatments. Assuming a traditional model that specifies a first-order carryover effect, this study investigates the following alternative crossover trial designs: (1) two-treatment two-period four-sequence design (Balaam, 1968) design, (2) two treatments-three period-four sequence design (Ebbutt, 1984), and (3) three treatment-two period-six sequence design (Koch, 1983). Each design has attractive properties and, when properly applied, allows both treatment and carryover effects to be estimated.

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