Whenever the observations in an experimrnt must be made in somr time order sequence, there is a substantial likelihood that there may bc somo time order dcpendency in the results. In addition, it is frequently cxprnsivr to change levels for one or more of the factors in thr study. In this paper these two facts are documcntcd with personal case histories and a general method is proposed for taking these difficulties into consideration when planning an expcrimrnt. The method introduces Monte Carlo like techniques into the design of cxperimcnts.