To ensure the internal validity of the study, it is essential that the only
difference between the treatment conditions is the single variable that was
used to defi ne the conditions. In Figure 6.3, for example, the only difference
between the three rooms is the background music. If there is any other factor
that differentiates the treatment conditions, then the study has a confounding
variable and the internal validity is threatened. For example, if the pleasant
and calming music room is painted green, the no-music room yellow, and the
unpleasant and aggressive music room red, then the study is confounded. In
this case, the color of the room is a confounding variable. Specifi cally, any
differences in performance from one room to another may be explained by
the music but they also may be explained by room color. In the following
sections, we identify three different ways that internal validity can be
threatened. That is, we examine three different categories of confounding
variables: environmental variables, individual differences, and time-related
variables.