30 CHAPTER 1
random assignment
process of assigning subjects to the
experimental or control groups ran-
domly, so that each subject has an
equal chance of being in either group.
control group
subjects in an experiment who are
not subjected to the independent vari-
able and who may receive a placebo
treatment.
experimental group
subjects in an experiment who are
subjected to the independent variable.
had on the children’s behavior could be confused (confounded) with the possible effects of
the family background. The researchers wouldn’t know if the children were being aggres-
sive because they watched the cartoon or because they liked to play aggressively anyway.
The best way to control for confounding variables is to have two groups of partic-
ipants: those who watch the violent cartoon and those who watch a nonviolent cartoon
for the same length of time. Then the researchers would measure the aggressive behav-
ior in both groups. If the aggressive behavior is significantly greater in the group that
watched the violent cartoon (statistically speaking), then researchers can say that in this
experiment, violent cartoon watching caused greater aggressive behavior.
The group that is exposed to the independent variable (the violent cartoon in the
example) is called the experimental group, because it is the group that receives the
experimental manipulation. The other group that gets either no treatment or some kind
of treatment that should have no effect (like the group that watches the nonviolent car-
toon in the example) is called the control group because it is used to control for the possi-
bility that other factors might be causing the effect that is being examined. If researchers
were to find that both the group that watched the violent cartoon and the group that
watched the nonviolent cartoon were equally aggressive, they would have to assume
that the violent content did not influence their behavior at all.THE IMPORTANCE OF RANDOMIZATION As mentioned previously, random selection is
the best way to choose the participants for any study. Participants must then be assigned
to either the experimental group or the control group. Not surprisingly, random assign-
ment of participants to one or the other condition is the best way to ensure control over
other interfering, or extraneous, variables. Random assignment means that each partici-
pant has an equal chance of being assigned to each condition. If researchers simply looked
at the children and put all of the children from one day care center or one pediatrician’s
recommendations into the experimental group and the same for the control group, they
would run the risk of biasing their research. Some day care centers may have more natu-
rally aggressive children, for example, or some pediatricians may have a particular client
base in which the children are very passive. So researchers want to take the entire par-
ticipant group and assign each person randomly to one or the other of the groups in the
study. Sometimes this is as simple as picking names out of a hat.Watch the Video Experiments: Experimental Group versus Control GroupCC