Internal Validity
The degree to which a study establishes a trustworthy cause-and-effect relationship between a treatment and an outcome.
What is Internal Validity?
Internal validity depends heavily on the procedures of a study and how rigorously it controls for confounding variables. A study with high internal validity convincingly demonstrates that it was the independent variable, and not some extraneous factor, that caused the change in the dependent variable.
Why Internal Validity Matters
It is the fundamental requirement for making causal claims. Without it, researchers cannot know if their intervention actually worked.
Example
In a randomized controlled trial testing a new teaching method, the researcher tightly controls the curriculum, class time, and assigns students randomly, ensuring that differences in test scores are due to the method, not student background.
Common Mistakes
- Failing to account for maturation (participants changing naturally over time).
- Ignoring selection bias where the groups being compared were different to begin with.