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
and (c) a double-blind
by concealing the nature of treatment
from the subjects and the person taking measurements.
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