Blocks of similar experimental units

Experiments with matched pairs or matched groups are special kinds of randomised block experiments. In them, a collection of experimental units is grouped based on similar weights, ages, etc. In other situations, the experimental units naturally separate into groups.

Time
It may be impossible to conduct the whole experiment at once. Runs of the experiment at different times should be treated as blocks.
Position
Different experimental units may be located in different research stations, farms, fields in the same farm, or even benches in a greenhouse. These different locations should be treated as blocks.
Groups of individuals
Animals may need to be managed in different herds or kept in groups in a few cages. These groupings should be treated as 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.