Design of experiments is the esign of all information-gathering exercises where variation is present, whether under the full control of the experimenter or not. (The latter situation is usually called an observational study.) Often the experimenter is interested in the effect of some process or intervention (the 'treatment') on some objects (the 'experimental units'), which may be people. Design of experiments is thus a discipline that has very broad application across all the natural and social sciences. It is also called experimental design at a slight risk of ambiguity (it concerns designing experiments, not experimenting in design).