ABSTRACT
This chapter considers how to select people for the experimental and control groups of an intervention study. There is a big literature on methods of allocation to intervention. The main principle behind modern intervention studies is that allocation should be completely random, i.e., not predictable from any characteristics of participants. Suppose the experimenter allocates aphasic patients to a study to improve word-finding according to some systematic method such as their serial order on a list, but notices that the next person destined for the intervention group has particularly profound difficulties with comprehension. In theory this might seem sufficient, but it ignores one aspect of real life: people will sign up for a trial but then drop out before assignment of intervention. However, the permuted blocks design has a downside; it is relatively easy for the researcher to see a pattern emerge in the allocation, so the randomization can become predictable.
