What is Internal validity?
Internal validity is the extent to which a study can establish a causal relationship between the treatment and the outcome, ruling out alternative explanations.
How it works
Threats to internal validity include confounding variables, selection bias, maturation (participants change over time regardless of treatment), history (external events influence outcomes), testing effects (the measurement itself changes behavior), and regression to the mean. Randomized controlled trials maximize internal validity by ensuring that treatment and control groups differ only in the treatment received. However, maximizing internal validity often requires artificial conditions that reduce ecological validity.
Applied example
A study showing that students who use a new app score higher on exams may lack internal validity if students self-selected into the app group, because motivated students who chose to use the app would have scored higher regardless.
Why it matters
Internal validity is the first requirement of useful research: before asking whether findings generalize, you must know whether they are real.
