An alternative to having a different group in each treatment condition is to have
the same group of individuals participate in all of the different treatments. In
Figure 6.3, for example, the researcher could test the same group of people in
all three of the background-music conditions. The basic problem with this type of research is that it not only compares scores obtained in different treatments,
but also compares scores obtained at different times. For example, a group of
students could be tested in the pleasant and calming music room on Monday, in
the unpleasant and aggressive music room on Tuesday, and then brought back
to be tested again in the no-music room on Wednesday. Although the background
music changes from day to day, there are a number of other variables
that also change as time goes by. It is possible that these other time-related variables
could be confounding variables. That is, during the time between the fi rst
treatment condition and the fi nal treatment condition, individual participants
or their scores may be infl uenced by factors other than the treatments. Any
factor affecting the data other than the treatment is a threat to the internal
validity of the study. Note that time-related variables can be environmental,
such as the weather or time of day, and they can be participant variables, such
as mood or physical state. In this section, we identify fi ve time-related threats to
internal validity.