The sample size is the number of subjects or individuals
studied. In scientific literature, the letter "N" (or "n")
is used to designate sample size. In experiments, scientists usually
work with a sample of subjects rather than the full population.
Good experimental design must include a sample of subjects that
is representative of the greater population being studied.
Scientific hypotheses are tested by contrasting a "control
group" with an "experimental group". In experimental
groups, the variable under study is manipulated. In control groups,
the variable under study is not manipulated. Control groups are
important because they serve as a reference point for the experimental
groups. In some studies, subjects are studied twice, once with the
experimental manipulation, and once with a control manipulation.
In this case, the subject's control measurements can be compared
to their own experimental measurements.
Scientists examining human subjects often use placebos in their
control treatments. A placebo is a substance or treatment
that replicates the experimental treatment but lacks the active
component. Placebos give the control subjects the same expectation
as the experimental subjects and thus enable scientists to control
for psychological effects.
Scientists often need to replicate an experiment several
times to establish a clear relationship between control and experimental
groups. Each performance of the experiment is called a replicate.
The number of replicates that a scientist chooses to perform is
important; too few replicates could lead to inconclusive results
while too many replicates is a waste of resources.
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