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IDENTIFYING THREATS TO INTERNAL VALIDITY 155

tending to score closer to the mean is known as “regressing toward the mean” (sta-
tistical regression). Likewise, those with very high abilities would also have a
greater tendency to regress toward the mean—they will score lower on the posttest
than on the pretest. Thus, those who are at either end of the continuum with
respect to a variable would not “truly” reflect the cause-and-effect relationship. The
phenomenon of statistical regression is thus yet another threat to internal validity.

Mortality
Another confounding factor on the cause-and-effect relationship is the mortality
or attrition of the members in the experimental or control group or both, as the
experiment progresses. When the group composition changes over time across
the groups, comparison between the groups becomes difficult, because those
who dropped out of the experiment may confound the results. Again, we would
not be able to say how much of the effect observed arises from the treatment,
and how much is attributable to the members who dropped out, since those who
stayed with the experiment could have reacted differently from those who
dropped out. Let us see an example.

Example 7.2 A sales manager had heard glowing reports about three different training pro-
grams that train salespersons in effective sales strategies. All three were of 6
weeks’ duration. The manager was curious to know which one would offer the
best results for the company. The first program took the trainees daily on field
trips and demonstrated effective and ineffective sales strategies through practical
experience. The second program trained groups on the same strategies but
indoors in a classroom type of setting, lecturing, role playing, and answering
question from the participants. The third program used mathematical models and
simulations to increase sales effectiveness.
The manager chose eight trainees each for the three different programs and
sent them to training. By the end of the fourth week, three trainees from the first
group, one from the second group, and two from the third group had dropped
out of the training programs due to a variety of reasons including ill health, fam-
ily exigencies, transportation problems, and a car accident. This attrition from the
various groups has now made it impossible to compare the effectiveness of the
various programs.
Thus, mortality can also lower the internal validity of an experiment.

IDENTIFYING THREATS TO INTERNAL VALIDITY

Let us examine each of the possible seven threats to internal validity in the con-
text of the following scenario.
An organizational consultant wanted to demonstrate to the president of a company,
through an experimental design, that the democratic style of leadership best enhances