For example, becoming
fatigued in one treatment may lead to poorer performance in the next
treatment. You should realize that this problem can also be viewed as a
threat to internal validity. Specifically, the experience of being tested in
one treatment may explain why the participants’ scores are different in
the following treatment. Remember, an alternative explanation for an
observed difference is a threat to internal validity. In this case, the
researcher does not know whether the observed change in performance
is caused by the different treatments or by fatigue. Any possible change
in performance caused by participation in a previous treatment is called
a testing effect and is a threat to internal validity because it provides an
alternative explanation for the results.
It also is possible that a specific treatment causes changes in the
participants so that the lingering aftereffects of the treatment carry over
into the next treatment (or treatments) and alter the participants’ scores.
For example, participants in a memory study may learn a new rehearsal
strategy in one treatment condition, and continue to use the strategy to
help improve their memory scores when participating in later treatment
conditions. Appropriately, these effects are called carry-over effects.
Notice that a testing effect, such as fatigue, comes from general experience
in the research study, whereas carry-over effects are caused by
experiencing a specific treatment.