Interval Validityย 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 variableโs effects, providing strong evidence of a functional relationship.
Example
Imagine a study aiming to decrease disruptive classroomย behaviorย using a token economy system. The researcher implements the intervention, and over the course of several weeks, the disruptiveย behaviorย decreases. For strongย internal validity, the researcher needs to ensure that theย only factor influencing theย behaviorย 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.