Capture-recapture techniques. Quick and cheap.
Gutteridge
W
W
Collin
C
C
D000328:Adult; D003625:Data Collection; D006233:Persons with Disabilities / Q000706:statistics & numerical data*; D004739:England / Q000453:epidemiology; D005194:Family Practice; D006801:Humans; D008875:Middle Aged; D011159:Population Surveillance / Q000379:methods*