1) When many extraneous variables are ruled out by an experimental design, the research has high internal validity. That is, changes in the dependent variable may be reasonably attributed to changes in the independent variable (cause - effect). Internal validity is the minimum requirement for all experiments.

2) A feature that describes how correct or valid conclusions are about the functionality of a relationship between two variables, such as a procedure and changes in behavior. Internal validity, then, addresses the validity of the answer to the question, "did the treatment bring about the behaviour change?"