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A completely randomized (CR) design, which is the simplest type of the basic designs, may be defined as a design in which the treatments are assigned to experimental units completely at random, that is the randomization is done without any restrictions. The design is completely flexible, i.e., any number of treatments and any number of units per treatment may be used. Moreover, the number of units per treatment need not be equal. A completely randomized design is considered to be mo useful in situations where (i) the experimental units are homogeneous, (ii) the experiments are small such as laboratory experiments, and (iii) some experimental units are likely to be destroyed or to tail to respond.
Experimental Layout:
The layout of an experiment is the actual placement of the treatments on the experimental units, which may pertain Lo time, space or type of material. Suppose we have  treatment and the experimental material is divided into experimental units. We shall then assign the  treatments at random to the  experimental units in such a way that the treatment  is applied  times, with .When each treatment is applied the same number of times, then and . Usually, each treatment is applied (or replicated) an equal number of times.
An example of the experimental layout for a completely randomized design (CR) using four treatments  and  ,each repeated  times, is given below:
The result or response of a treatment which may be a real yield, the gain in weight, the ability, etc., is generally called yield and is represented by the letter .
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