Concurring with Johnstone’s work (1988, 1997), we believe that a level of randomness can be attained which, though not ideal, should be sufficient to allow the researcher to proceed, albeit with caution. Proper experimental design depends on stratification from which a ‘‘random’’ sample
within each stratum can be achieved under specified constant conditions. However, we are unable to document any explicit attention to this aspect of experiential design in the papers we reviewed.