Evidence-appraisal glossary
Internal validity
Internal validity is the degree to which a study's design and conduct support a genuine cause-and-effect link between the intervention and the measured outcome, rather than the result being produced by bias, confounding, or chance. High internal validity means you can trust the finding within that study.
Also called: Study validity, Freedom from bias.
What it is
Internal validity asks a simple question: within this one study, is the observed effect real, or could something else explain it? A study is internally valid when its result is unlikely to be an artifact of systematic error (bias), a lurking third variable (confounding), or random noise (chance).
How to use it when reading a study
Treat internal validity as the first gate. A finding that is not internally valid cannot be salvaged by a large sample or a fancy analysis.
- Bias: Check how participants were allocated, whether allocation was concealed, and whether patients, clinicians, and assessors were blinded. Look at dropout and whether analysis was intention-to-treat.
- Confounding: In non-randomized studies, ask what baseline differences between groups could drive the result and whether they were adjusted for.
- Chance: Weigh the effect size and confidence interval, not just the p-value.
Formal tools like Cochrane's risk-of-bias framework are structured internal-validity checklists. Keep it distinct from external validity (generalizability): a study can be internally sound yet apply poorly to your patient.
Read the full Reading the Evidence blog.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.