Evidence quality
Internal validity
How well a study design and conduct support a trustworthy answer for its own participants.
What it means
Definition
Internal validity. Internal validity concerns bias, confounding, measurement and analysis within the study. Without internal validity, broad applicability is secondary because the result may not be reliable even for the study sample.
What it can tell you
It can help clarify what sort of evidence or claim is being discussed.
What it cannot tell you
It cannot, by itself, prove that the headline is accurate or clinically important.
Headline trap
Why this matters in headlines
Poor randomisation or heavy loss to follow-up can weaken internal validity.