Subjects bring with them into the investigation unique
characteristics, some learned and some inherent. Examples include
sex, height, weight, color, attitude, personality, motor ability, and
mental ability. If assigning subjects to comparison groups results in
unequal distribution of these subject-related variables, then there
is a possible threat to internal validity. Suppose that subjects in
two comparison groups are unlike with respect to the independent
variable and one of these subject-related variables. If scores on the
dependent measure differ between the groups, the discrepancy may be
due to the independent variable or to the
subject-related variable.
Background Information
Example
It so happened that there were an equal number of boys
and girls in the classes, so for convenience the boys were assigned
to the Control Group and the girls to the Experimental Group. One day
at school, the boys were told to go to one room and the girls to
another room, where they were exposed to their respective conditions.
Two days later, the Generalization Probe was conducted. The mean
score for children in the Control Group was 1.2 and the mean score
for children in the Experimental Group was 3.4. We conclude that the
20-minute interactive video improved the children's self-protection
skills in a potential abduction situation.
Nonexample
The name of each child in the classes was written on a separate slip
of paper. All the slips were put in a bowl and mixed up thoroughly.
Students were assigned to the Experimental Group and to the Control
Group alternately as their names were pulled out of the bowl one at a
time. One day at school, the children in the Control Group were told
to go to one room and the children in the Experimental Group to
another room, where they were exposed to their respective
conditions. Two days later, the Generalization Probe was
conducted. The mean score for children in the Control Group was 1.2
and the mean score for children in the Experimental Group was 3.4. We
conclude that the 20-minute interactive video improved the children's
self-protection skills in a potential abduction situation.
Analysis
The first item is an example in which
selection is a threat to internal validity.
The children in the two comparison groups are unlike with respect to
whether or not they viewed the interactive video and with respect to
gender. The higher Generalization Probe score by the Experimental
Group may be due to exposure to the interactive video
or to the sex of each child.
In the second item, by randomly allocating subjects to conditions,
the only way sex (and other subject-related variables) can be
unevenly distributed between the two comparison groups is through
chance. Thus, the children in the two groups appear to be unlike only
with respect to whether or not they viewed the interactive video. We
can be more confident that the better Generalization Probe score by
the Experimental Group was not the result of
selection.
