Blocks of similar experimental units
Experiments with matched pairs or matched groups are special kinds of randomised block experiments. In some experiments, the grouping of experimental units is a fairly subjective process, perhaps based on similar weights, ages, etc. In other situations, the experimental units are naturally grouped into blocks.
If the number of experimental units in each block is a multiple of the number of factor levels in the experiment, it is possible to randomly allocate each factor level the same number of times within each block. The resulting experiment is called a randomised block design.
Acupuncture and Codeine for dental pain relief
In this experiment, the blocks of individuals arose as matched groups.
Clotting time of plasma
The blocks of experimental units in the next example occurred naturally.
Effect of cultivar on plant height
In this experiment, the blocks were also distinct groups of units.