Evidence quality
Selection bias
Bias caused by who enters, remains in or is analysed in a study.
What it means
Definition
Selection bias. Selection bias occurs when the people studied differ meaningfully from the people the headline implies. It can make benefits or harms look larger, smaller or different from real-world practice.
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
An app-based health survey may miss older or less digitally connected patients.