The Two-Period Change-Over Design and Its Use in Clinical Trials

Some properties of the two-period change-over design are investigated. It is shown that if the effect of subjects is assumed to be a random variable, the difference between the direct effects and the differenice between the residual effects of treatments are estimable, but the difference between periods is not. The amount of experimentation necessary to achieve a specified power of the test of equality of the direct effects of treatments resulting from a two-period change-over design is compared to the amount required for a design in which the subjects are assigned randomly to a single treatment. This comparison shows that the two-period changeover design is preferable when the residual effects of the treatments are equal and the correlation between the response to the two tieatments is positive. Otherwise the design in which there is random assignment to a single treatment is preferable.