Randomized Block Design:
It is improvement over the C. R design. In the R. B design the principle of local control can be applied along with the other two principles of experimental designs. In the R. B. Design subjects are first divided into groups known's as blocks such that within each group the subject are relatively homogenous in respect to some selected variable. The variable selected for grouping the subjects is one that is believed to be related to the measures to be obtained in respect of the dependent variable. The number of subjects in a given block would be equal to the number of treatment sand on subject in each block would be randomly assigned to each treatment. In general blocks are the levels at which we hold the extraneous factor fixed so that it contribution to the total variability of data can be measured. The main feature of the R. B design is that in this each treatment appears the same number of times of each block. The R. B. Design is analyzed by the two way analysis of variance ( two way ANOVA) technique.
If each student separately randomized the order in which he or she took the four tests ( by using random numbers or some similar device) we refer to the design of this experiment as a R.B design. The purpose of this randomization is to take care of such possible extraneous factor (Say as fatigue) or perhaps the experience gained from repeatedly taking the test.