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b. History
Outside events may influence subjects in the course of
the experiment or between repeated measures of the dependent
variable. Suppose that the dependent variable is measured twice for a
group of subjects, once at Time A and later at Time B, and that the
independent variable is introduced in the interim. Suppose also that
Event A occurs between Time A and Time B. If scores on the dependent
measure differ at these two times, the discrepancy may be due to the
independent variable or to Event A.
Background Information
Example
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. Immediately afterwards, while walking back to their
regular classroom, all the children in the Control Group saw a man
laughing and joking with their school principal. Two days later, the
Generalization Probe was conducted, during which many of the Control
Group children recognized the stranger as the man who made their
principal laugh. 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. The rooms were adjacent to each other, and, when the
special class was over, the two groups left their rooms at exactly the
same time. Immediately afterwards, while walking back to their regular
room, some of the children saw a man laughing and joking with their
school principal. Two days later, the Generalization Probe was
conducted, during which some of the children recognized the stranger
as the man who made their principal laugh. It appears that the number
who did so was equally proportioned between the two groups. 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
history 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
another event encountered in the course of the experiment, that being
whether or not they saw the confederate laughing and joking with
their school principal. The higher Generalization Probe score by the
Experimental Group may be due to exposure to the interactive video
or to the fact that only the Control Group subjects
witnessed this other event. Seeing the confederate laugh and joke
with their school principal may have made him a less intimidating
figure for the Control Group subjects, which, in turn, may have
caused them to be less likely to verbally refuse and run away from
him on the subsequent Generalization Probe.
In the second item, the experimenter attempted to control for
differential exposure to outside influences by treating the two
comparison groups as equally as possible other than which video she
showed them. While some of children saw the confederate laughing and
joking with their school principal, in this case it appears that the
number who did so was equally proportioned between the two groups. If
this is true, then we can be more confident that the better
Generalization Probe score by the Experimental Group was not the
result of history.
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