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Simple experimental designs invoking treated and control areas, with all treatments initiated at the same time, should not be used to assess transient responses to management actions or environmental disturbances. Such designs will not properly control for "time–treatment" interactions, involving differential responses of treated areas to nonrandom trends in the experimental environment. For example, survival trends for hatchery salmon stocks cannot be simply compared with survival trends in wild stocks, since the hatchery stocks may be more susceptible to changes in environmental factors such as ocean temperature. To control for such time–treatment interactions, it is suggested to use a "staircase" experimental design in which treatment is initiated at different times on the treated areas. Computation of the average transient and interaction response parameters for such designs, while correcting for temporal trends and inherent differences among areas, can be done using general linear models. The optimum design configuration (number of treated versus control areas, number of pretreatment control times, etc.) involves spreading the treatment starts widely over time and using relatively few control areas.
Walters et al. (Tue,) studied this question.