Subject Variables

The experimenter might want to control or hold constant other variables.
Perhaps she suspects that age, social class, ethnicity, and gender could also
influence the children’s aggressiveness. She might want to make sure that
these subject variables are eliminated by either choosing subjects who are
alike in these ways or by making sure that the groups are balanced for these
factors (for example, equal numbers of boys and girls in each group). There
are numerous other extraneous variables that might concern the researcher,
including the time of day when the children participate, the length of time
between television viewing and the assessment of aggressiveness, the children’s
diets, the children’s family structures (single versus dual parent, siblings
versus only child), and the disciplinary styles used in the homes. Resource
limitations prevent every extraneous variable from being controlled,
yet the more control, the more confident the experimenter can be of the
cause-effect relationship between the independent and dependent variables.