Download Positive vs Negative controls

POSITIVE vs NEGATIVE
EXPERIMENTAL CONTROL
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POSTIVE CONTROL:
~ not exposed to the experimental treatment but that is exposed to
some other treatment that is known to produce the expected effect
~ Ensures that there is an effect when there should be an effect
~ If the positive control does not produce the expected result, there
may be something wrong with the experimental procedure, and the
experiment is repeated.
NEGATIVE CONTROL:
~ not exposed to the experimental treatment or to any other treatment
that is expected to have an effect.
~ Ensure that there is no effect when there should be no effect.
~group where nothing is expected to happen
EXAMPLE:
Experiment to determine if some lettuce was contaminated with
bacteria. You wipe lettuce leaves with a swab, wipe the swab on a
bacterial growth plate, incubate the plate, and see what grows on the
plate.
NEGATIVE CONTROL:
Wipe a sterile swab on the growth plate. You would not expect to see
any bacterial growth on this plate. If you do, it is an indication that your
swabs, plates, or incubator are contaminated with bacteria that could
interfere with the results of the experiment.
POSITIVE CONTROL: Wipe a swab on an existing colony of bacteria
and wipe it on the growth plate. Shows your experimental set up will
grow bacteria.
EXAMPLE:
Experiment to determine if a new drug has an impact on lowering blood
pressure. Test subjects with high blood pressure are given a pill and
percentage of patients whose blood pressure decreased in measured.
NEGATIVE CONTROL:
Test subjects are untreated or given a placebo pill.
If the group receiving the new drug shows improvement, it needs to be
compared to the placebo group. The treatment is only effective if the
treatment group shows more improvement than the placebo group.
POSTIVE CONTROL:
Test subjects are given a drug that is known to reduce blood pressure.