Definition
Interval Validity refers to the degree to which one can confidently conclude that the changes observed in the dependent variable are directly caused by the manipulation of the independent variable, rather than by other extraneous variables. High internal validity means that the study design effectively isolates the independent variable’s effects, providing strong evidence of a functional relationship.
Example
Imagine a study aiming to decrease disruptive classroom behavior using a token economy system. The researcher implements the intervention, and over the course of several weeks, the disruptive behavior decreases. For strong internal validity, the researcher needs to ensure that the only factor influencing the behavior is the token economy intervention. This can be done by controlling for other variables, such as ensuring no other classroom rules were changed, no additional instructions or consequences were introduced by the teacher, and the class environment remained consistent.