What is an Experiment?
In an experiment, a researcher manipulates one or more variables,
while holding all other variables constant. By noting how
the manipulated variables affect a response variable,
the researcher can test whether a causal relationship exists
between the manipulated variables and the response variable.
Parts of an Experiment
All experiments have independent variables, dependent variables, and
experimental units.
- Independent variable. An independent variable
(also called a factor) is an explanatory
variable manipulated by the experimenter.
Each factor has two or more levels (i.e.,
different values of the factor). Combinations of factor
levels are called treatments. The table
below shows independent variables, factors, levels, and
treatments for a hypothetical experiment.
|
|
Vitamin C |
|
|
0 mg |
250 mg |
500 mg |
Vitamin E |
0 mg |
Treatment 1 |
Treatment 2 |
Treatment 3 |
400 mg |
Treatment 4 |
Treatment 5 |
Treatment 6 |
In this hypothetical experiment, the researcher is studying the
possible effects of Vitamin C and Vitamin E on health. There
are two factors - dosage of Vitamin C and dosage of Vitamin E.
The Vitamin C factor has three levels - 0 mg per day, 250 mg
per day, and 500 mg per day. The Vitamin E factor has 2 levels -
0 mg per day and 400 mg per day. The experiment has six
treatments. Treatment 1 is 0 mg of E and 0 mg of C, Treatment
2 is 0 mg of E and 250 mg of C, and so on.
- Dependent variable.
In the hypothetical experiment above, the researcher is looking
at the effect of vitamins on health. The dependent variable in
this experiment would be some measure of health (annual
doctor bills, number of colds caught in a year, number of
days hospitalized, etc.).
- Experimental units. The recipients of
experimental treatments are called experimental units.
The experimental units in an
experiment could be anything - people, plants, animals, or even
inanimate objects.
In the hypothetical experiment above, the experimental units would
probably be people (or lab animals). But in an experiment to measure
the tensile strength of string, the experimental units might be
pieces of string. When the experimental units are people, they are
often called participants; when the experimental units are animals,
they are often called subjects.
Characteristics of a Well-Designed Experiment
A well-designed experiment includes design features that
allow researchers to eliminate extraneous variables as an
explanation for the observed relationship between
the independent variable(s) and the dependent variable.
Some of these features are listed below.
Confounding
Confounding occurs when the experimental controls
do not allow
the experimenter to reasonably eliminate plausible alternative
explanations for an observed relationship between independent
and dependent variables.
Consider this example. A drug manufacturer tests a new cold
medicine with 200 participants - 100 men and 100 women.
The men receive the drug, and the women do not. At the end
of the test period, the men report fewer colds.
This experiment implements no controls! As a result,
many variables are confounded, and it is impossible to say
whether the drug was effective. For example, gender is
confounded with drug use. Perhaps, men are less vulnerable
to the particular cold virus circulating during the
experiment, and the new medicine had no effect at all. Or
perhaps the men experienced a placebo effect.
This experiment could be strengthened with a few controls.
Women and men could be randomly assigned to treatments. One
treatment group could receive a placebo, with blinding. Then, if
the treatment group (i.e., the group getting the medicine) had
sufficiently fewer colds than the control group, it would be
reasonable to conclude that the medicine was effective in
preventing colds.
Test Your Understanding
Problem
Which of the following statements are true?
I. Blinding controls for the effects of confounding.
II. Randomization controls for effects of lurking variables.
III. Each factor has one treatment level.
(A) I only
(B) II only
(C) III only
(D) All of the above.
(E) None of the above.
Solution
The correct answer is (B). By randomly assigning experimental units to
treatment levels, randomization spreads potential effects of
lurking variables
roughly evenly across treatment levels.
Blinding
ensures that participants in control and treatment conditions
experience the
placebo effect equally, but it does not guard against
confounding. And finally, each
factor
has two
or more treatment levels. If a factor had only one treatment
level, each participant in the experiment would get the same treatment
on that factor. As a result, that factor would be
confounded with every other factor in the experiment.