Outliers from sampling error: another
cause of outliers is sampling. It is
possible that a few members of a
sample were inadvertently drawn from
a different population than the rest of
the sample. For example, in education,
in advert entry sampling academically
gifted or mentally retorted students is
a possibility and (depending on the
goal of the study) might provide
undesirable outliers. These cases
should be removed as they do not
reflect the target population.