A formal experiment has several fixed parts, regardless of
what the experiment is about. First the experimenter must create a hypothesis, which is
a testable theory from which the experiment is derived. Then an experimental population
is chosen. This could be people or other living things, objects, or even atoms or
molecules; the key is that no matter what the experimental population is composed of, is
should be a reasonably large, uniform sample, and should be
randomly divided into as many groups as there are levels of
treatment, plus one for the control.
The treatment is the
single variable or factor that the experimenter wishes to test. The control is a group
that is not subject to the treatment. The control group is considered "normal" and is
used as a baseline to compare the experimental group(s) to. It is important that only
the factor being tested is different from group to group, and everything else is held
constant.
Some treatments are done once, some are done in
increments. For example, an experimenter who wanted to examine the effect of vitamin C
supplementation on the mouse immune system could simply do a single treatment, in which
case there would be a group of mice who were fed a standard diet and a similar test
group who were fed the same diet plus a vitamin C supplement. Alternately, the
experiment could include a control and several test groups, with each test group getting
an incrementally larger supplementation of vitamin C.
As
the experiment proceeds, the same data is gathered by the same methods and at the same
frequency in each group. The data is then compared across groups by whatever statistical
analysis is appropriate, and conclusions regarding the correctness of the hypothesis are
drawn.
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