positive control is used in experiments to demonstrate that the
scientist and her/his laboratory are capable of carrying out the
classic toxicological experiment to determine the effect of compound
X will include three groups: a standard control group, a positive
control group, and the experimental group.
standard control group is the set of individuals (mice, cells,
people, etc.) that were exposed to a substance known to cause
positive control group is the set of individuals exposed to a
substance known to cause an effect, based on
previous, established and accepted research.
experimental group is exposed to compound X.
the experimenter to conclude that compound X has a real effect,
the impact of compound X should be statistically distinguishable
from that of the standard control.
the impact of X is not different, then the crucial question becomes
whether the impact of the positive control is different from the
control. If it is not, then the experiment has failed. No
conclusion can be reached about the impact of X because the positive
control should have been different from the control.
The implication is that there has been a procedural error or design
flaw in the experiment.
are many possible sources of such mistakes. For example:
unknown contaminant may be present, either preventing the
response altogether or provoking it in all groups simultaneously.
For example, if the control group is affected by the unknown
contaminant in a way that mimics the expected result of the
positive control, then the effect in the positive control
and the experimental group may be indistiguishable from the
laboratory procedures may be too sophisticated for the lab
conducting the studies because of a lack of training, inadequacy
of equipment, etc.
sample sizes used in the experiment may be too small to yield
statistically significant results.
the cause, experiments in which the positive control fails cannot
be used to reach conclusions about the impact of the experimental