Evidence-appraisal glossary
Number Needed to Screen
Number needed to screen is how many people must go through a screening program to prevent one bad outcome, such as one death from the targeted disease, over a given period.
Also called: NNS.
It recasts a screening trial's benefit into a tangible count, the screening cousin of number needed to treat. A large number signals that most participants gain nothing while still facing the harms of screening, including false positives and overdiagnosis. Reading it alongside those harms gives a fuller picture than a relative risk reduction, which can make small absolute benefits sound large.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.