The Completely Randomized Design

The completely randomized design (CRD) is the simplest of all experimental designs, both in terms of analysis and experimental layout. Here, treatments are randomly allocated to the experimental units entirely at random. Thus if a treatment is to be applied to five experimental units, then each unit is deemed to have the same chance of receiving the treatment as any other unit. The CRD is often used if we believe that the experimental material is homogeneous or uniform. In this case, the experimental units are regarded as a group and the investigator believes that the experimental material available contains only nonassignable variation, and that it would be impossible to try to group the material into blocks or some other subgroups such that the variation among subgroups is larger than among units within subgroups as far as the response variable under investigation is concerned. Usually, though not necessarily, the random assignment is restricted in such a manner as to have an equal number of experimental units assigned to each treatment. The CRD should therefore be used where extraneous factors can easily be controlled, such as in laboratories or green houses. The CRD is usually the choice design in pilot studies where experimental units and conditions are homogeneous.