• In an experiment, one or more treatments are imposed on the experimental units or subjects. Each treatment is a combination of levels of the explanatory variables, which we call factors. Outcomes are the measured variables that are used to compare the treatments.
• The design of an experiment refers to the choice of treatments and the manner in which the experimental units or subjects are assigned to the treatments.
• The basic principles of statistical design of experiments are comparison, randomization, and repetition.
• The simplest form of control is comparison. Experiments should compare two or more treatments in order to prevent confounding the effect of a treatment with other influences, such as lurking variables.
• Randomization uses chance to assign subjects to the treatments. Randomization creates treatment groups that are similar (except for chance variation) before the treatments are applied. Randomization and comparison together prevent bias, or systematic favoritism, in experiments.
• You can carry out randomization by giving numerical labels to the experimental units and using a table of random digits to choose treatment groups.
• Repetition of the treatments on many units reduces the role of chance variation and makes the experiment more sensitive to differences among the treatments.
• Good experiments require attention to detail as well as good statistical design. Many behavioral and medical experiments are double-blind. Lack of realism in an experiment can prevent us from generalizing its results.
• In addition to comparison, a second form of control is to restrict randomization by forming blocks of experimental units that are similar in some way that is important to the response. Randomization is then carried out separately within each block.
• Matched pairs are a common form of blocking for comparing just two treatments. In some matched pairs designs, each subject receives both treatments in a random order. In others, the subjects are matched in pairs as closely as possible, and one subject in each pair receives each treatment.