Research Methods ❮ 65Confounding variables limit confidence in research conclusions. All participants, also called
subjects, attend the same two sessions upon which the quiz is based. The experimental
group is permitted to study for the quiz for 1 hour in the evening before going to sleep
while the control group watches an unrelated comedy show. The control group studies for
the quiz for 1 hour in the morning after awakening. The experimental group watches the
comedy show in the morning at the same time. Everyone eats breakfast together, and then
they all take the same quiz. If the experimental group scores significantly higher than the
control group, the experimenter can say that the results support the hypothesis. How does
the experimenter measure effectiveness of studying? The experimenter uses the score on the
quiz as the operational definition of effectiveness of studying. An operational definition
describes the specific procedure used to determine the presence of a variable.
In order to attribute a particular result to a specific factor, the controlled experiment
must limit variables. Confounding variables that could contribute to the effect must be
eliminated. Participants in the biology quiz experiment need to share the same environ-
mental factors; they need to eat the same foods, sleep in similar beds in the same rooms,
sleep for the same amount of time, etc.
Eliminating Confounding Variables
Experimenter bias (also called the experimenter expectancy effect) is a phenomenon
that occurs when a researcher’s expectations or preferences about the outcome of a study
influence the results obtained. This is a special kind of confounding variable that can occur
when a researcher is unaware that he or she is treating either the experimental group or
control group differently from the other. A simple smile when addressing the experimental
group that is not also shown to the control group qualifies as experimenter bias and as
a confounding variable. The clues participants discover about the purpose of the study,
including rumors they hear about the study suggesting how they should respond, are called
demand characteristics. To eliminate the effects of demand characteristics, experimenters
use the single-blind procedure, a research design in which the participants don’t know
which treatment group—experimental or control—they are in. To eliminate the effects of
both experimenter bias and demand characteristics, experimenters use the double-blind
procedure, a research design in which neither the experimenter nor the participants know
who is in the experimental group and who is in the control group. The double-blind is
most easily accomplished when a second experimenter or assistant who doesn’t know the
hypothesis or group assignments administers the experiment, keeping the principal inves-
tigator away from the subjects. When a number of factors might be responsible for an
observed effect, to determine which deserves the credit, an experimenter needs to system-
atically manipulate or vary one or more factors while holding constant all the others that
might be important. The effects of these manipulated events on some behavioral reaction
are then assessed. It is then possible to demonstrate whether one factor is responsible for
the result or whether an interacting package of factors is involved.
In experiments involving drugs, participants in the experimental group usually receive
the drug with the active ingredient, while subjects in the control group receive a drug that
seems identical, but lacks the active ingredient. The imitation pill, injection, patch, or
other treatment is called a placebo. Subjects sometimes believe that the treatment will be
effective, and they think they experience an improvement in health or well-being. This is
the placebo effect. The placebo effect is now used to describe any cases when experimental
participants change their behavior in the absence of any kind of experimental manipula-
tion. The experiments need not involve drugs at all.
A research design that uses each participant as his or her own control is called a within-
subjects design. For example, the behavior of an experimental participant before receiving
treatment might be compared to his or her behavior after receiving treatment. Two treatments