Heather’s experiment was in real trouble because she ran all subjects getting treatment B on a Friday and we all know that Fridays are different! The A group was tested under quiet conditions; the B group was confounding in Heather’s experiment because the testing conditions changed along with the independent variable. Her problem could have been avoided by using one of the three general techniques for controlling physical variables; elimination, constancy of conditions, and balancing. We cannot possibly identify all the extraneous variables that influence the outcome of a study but we try to find as many as we can. By using control techniques, we increase the chances of an internally valid experiment.