Concepts underlying experiments

This guide describes the main concepts that are involved in designing an experiment. Its direct use is to assist scientists involved in the design of on-station field trials, but many of the concepts also apply to the design of other research exercises. These range from simulation exercises, and laboratory studies, to on-farm experiments, participatory studies and surveys. What characterises the design of on-station experiments is that the researcher has control of the treatments to apply and the units (plots) to which they will be applied. The results therefore usually provide a detailed knowledge of the effects of the treat-ments within the experiment. The major limitation is that they provide this information within the artificial "environment" of small plots in a research station. A laboratory study should achieve more precision, but in a yet more artificial environment. Even more precise is a simulation model, partly because it is then very cheap to collect data. The limitation here is that the "model" is not "real": the results are limited by the parameters of the model and the extent to which it correctly represents reality. It is easier to generalise from a well-planned on-farm trial in so far as the farms represent the recommendation domain, but there is less control over the treatments and plots. A survey makes generalisation easy, but with no control of the interventions to be explored. A participatory exercise gives detailed observations in a natural setting. A typical research project will involve a range of objectives that lead to a number of these types of studies.

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