We often want to compare two populations on the basis of experiment. For example, a researcher wants to test the effect of new drug on blood pressure. In the experiment an improvement may have been often due to the placebo effect when a participant (a subject) of the treatment believes that he or she has been given an effective treatment. To protect against such biases, the design of experiment should consider (a) the use of a control group in which the subjects are given a placebo, and an experimental group in which the subjects are treated with the new drug, (b) the randomization by assigning the subjects between the control and the experimental groups randomly, and (c) a double-blind experiment by concealing the nature of treatment from the subjects and the person taking measurements.

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