Confounding is when a researcher does not control some extraneous variables that may influence the results...the only variable that should influence the results is the variable being studied. If a variable other than the one that is manipulated by the researcher has any affect at all on the measurements, then the study is said to be confounded. This is a very serious problem since the researcher can't really claim that he/she established cause and effect. If the researcher is studying the effect of some pain medication (drug A) on the reduction of pain, but the researcher fails to control for participants taking other medications at the same time, how can the researcher claim that the results are due to the pain medication (drug A) or the other drugs that participants took? When the researcher controls all extraneous variables and can claim that only the variable that was manipulated has any effect on the results, the study is said to have internal validity.