Clinicians often overestimate their ability to reach the individuals they intend to serve. Clinic-based vaccinators, for example, are usually surprised by the results of household surveys that knock several points off their vaccination coverage data. While discrepancies can arise because of errors in the numerator (how many receive a vaccine) or the denominator (how many are eligible for the vaccine), the second measurement is more commonly contested. Clinicians grumble about survey definitions and procedures and claim they know their clients better than anyone else; surveyors laugh at coverage estimates in excess of 100% that sometimes grace clinic bulletin boards.
Tuli K. Epidemiology and the Delivery of Health Care Services: Methods and Applications. JAMA. 2010;303(15):1541-1542. doi:10.1001/jama.2010.465