4 - Blocking

Summary

Types of block

If the plots are not all reasonably similar, we should group them together into blocks in such a way that plots within each block are alike.

There are three main types of block.

Natural discrete divisions

These divisions between the experimental units are already present. If the experimental units are new-born animals then litters make natural blocks. In an experiment on people or animals, the two sexes make obvious blocks. In testing tags on cows' ears, the ears are the experimental units and cows are the blocks. In an industrial process, a block could be a batch of chemical or ore used for part of the process.

Example (Insect repellent) Midges in Scotland are a severe irritant in July and August. A researcher wants to try out some insect repellents, which are applied to people's skin. Twelve people volunteer for the experiment. It is known that people vary widely in their attractiveness to midges, so the researcher uses people as blocks, applying different repellents to each arm. After a fixed period of exposure to midges, the number of midge bites on each arm of each volunteer is recorded.

Sometimes there is more than one type of natural discrete block. If the experimental units are half-leaves of tobacco plants then whole leaves make one sort of block while the plants make another. In a consumer experiment, such as Example 1.11, testers and weeks are both natural blocks. In an experiment in the laboratory, technicians, benches and days may all be blocks.

If an experiment is carried out on plots that had previously been used for another experiment then you should consider whether to deem the previous treatments to be blocks.