StATS: What is an experimental design?

A research design where the researchers control the allocation of a treatment to the research subjects. Contrast this with an observational design, where the researchers do not have this control. Information from an experimental design is generally considered more authoritative than information from an observational design. For example, randomization is possible when the authors control group membership. Randomization provides some level of assurance that the two groups are comparable in every way except for the therapy received. Here are two examples of experimental designs:

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