Confounds: Threats to Validity

Why Are They Important?

Let's do a quick review of three common types of validity: Internal, External and Construct.

1. Internal Validity: When you think about internal validity, think INSIDE the experiment. Is your experiment so well designed that when the results are in, you feel confident that you can make truthful and definite statements about what happened in your study? If your study is relatively free of confounds; you will have high confidence in its results. That's internal validity.
2. External Validity: Can your results be generalized to people outside of your study?
3. Construct Validity: You are manipulating and measuring many concepts in your study – are you really tapping into these concepts?

• Most important to this discussion is Internal Validity and the concept of a confound.
• Most important to the concept of a confound is the concept of the independent variable.
• Independent Variable: The independent variable is the factor you manipulate. However, are you sure that the defined independent variable is the only factor influencing your dependent variable (the behavior that you measure or some other response measure)?
• If not, you have a confound. You have to think these issues through to make sure that you are truly (validly) manipulating the concept (or "construct"). No other factor should be operative.