looks for disease in people who feel well.
That single fact explains why the numbers work differently from ordinary medicine.
A patient with has already been selected by the illness.
A screened population has not, so most positive results are false.
Every leads to further tests, some of which carry real risk.
A second and less familiar harm is .
Some cancers found by screening would never have caused in a whole life.
Treating them cannot help the patient and can certainly cause damage.
statistics cover this up, because finding a slow cancer earlier makes survival look longer.
The only honest measure is whether deaths from that disease fall in the screened group.
On that measure some programmes succeed clearly and others do not.
Good practice is therefore to publish both numbers together in plain language.
A note that states the benefit without the damage is not information; it is .