One or more variables not under the control of the experimenter that vary systematically with the independent variable, decreasing the experimenter's ability to isolate cause and effect.

Factors that can cause or prevent the outcome of interest, are not intermediate variables, and are not associated with the factor(s) under investigation. They give rise to situations in which the effe [..]

Variables that cause/prevent an outcome from occurring outside of or along with the variable being studied. These variables render it difficult or impossible to distinguish the relationship between th [..]