Number 14/Life & ecology/Screening
The Overdiagnosis Fraction
The quantity
The fraction of screen-detected cases that would never have caused symptoms in the patient's lifetime.
01Why it matters
It is the quantity that decides whether a screening programme helps or harms, and it is rarely estimated because it requires long follow-up of unscreened controls.
02Expected value
Substantial for many cancers, small for conditions that always progress.
03How it would be measured
Compare cumulative incidence between screened and unscreened cohorts over decades.
04What would refute it
A fraction near zero would mean screening finds only progressive disease.