Design of Experiments:

A Design of Experiment (DOE) is a structured, organized method for determining the relationship between factors affecting a process and the output of that process.
Design of experiments is the design of all information-gathering exercises where variation is present, whether under the full control of the experimenter or not. Often the experimenter is interested in the effect of some process or intervention on some objects, 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).
Early examples of Experimental Design
In 1747, while serving as surgeon on HM Bark Salisbury, James Lind, the ship's surgeon, carried out a controlled experiment to discover a cure for scurvy.
Lind selected 12 men from the ship, all suffering from scurvy, and divided them into six pairs, giving each group different additions to their basic diet for a period of two weeks. The treatments were all remedies that had been proposed at one time or another. They were
• A quart of cider per day
• Twenty five gutts of elixir vitriol three times a day upon an empty stomach,
• Half a pint of seawater every day
• A mixture of garlic, mustard and horseradish, in a lump the size of a nutmeg
• Two spoonfuls of vinegar three times a day
• Two oranges and one lemon every day.
The men who had been given citrus fruits recovered dramatically within a week. One of them returned to duty after 6 days and the other became nurse to the rest. The others experienced some improvement, but nothing was comparable to the citrus fruits, which were proved to be substantially superior to the other treatments.
In this study his subjects' cases "were as similar as I could have them", that is he provided strict entry requirements to reduce extraneous variation. The men were paired, which provided replication.
From a modern perspective, the main thing that is missing is randomized allocation of subjects to treatments.