Therefore, the key to achieving internal validity is to ensure that
no extraneous variable is allowed to become a confounding variable. Because
every research study involves thousands of extraneous variables,
avoiding a confounding variable can be quite a task. Fortunately, however,
confounding variables can be classifi ed in a few general categories that make
it somewhat easier to monitor them and keep them out of a research study.
Before we examine the different categories of confounding variables, we
look more closely at the general structure of a research study for which internal
validity is a concern.