Table 3.

Common sources of bias in study design.

Type of bias	Description
Verification bias	Non-random selection for definitive assessment for disease with the old standard reference test
Errors in the reference	True disease status is subject to misclassification because the gold standard is imperfect
Spectrum bias	Types of cases and controls included are not representative of the population
Test interpretation bias	Information is available that can distort the diagnostic test
Unsatisfactory tests	Tests that are uninterpretable or incomplete do not yield a test result
Extrapolation bias	The conditions or characteristics of populations in the study are different from those in which the test will be applied
Lead time bias	Earlier detection by screening may erroneously appear to indicate beneficial effects on the outcome of a progressive disease
Length bias	Slowly progressing disease is over-represented in screened subjects relative to all cases of disease that arise in the population
Overdiagnosis bias	Subclinical disease may regress and never become a clinical problem in the absence of screening, but is detected by screening

Source: Pepe (2003).