Survival bias

This bias occurs when survivors of a highly lethal disease are more likely to enter a study than other cases.

Example: let's suppose we study the role of age as a potential risk factor for viral haemorrhagic fever (VHF), and that the study includes only those who are still alive at the time of the study. If older age is associated with VHF death, this will decrease the proportion of cases over a certain age in the study, and consequently underestimate the odds ratio. This is illustrated in the following table:

Age group	VHF survivors	VHF deaths	Total cases	Controls	ORs	ORtt
< 50 years	60	80	140	50	1.5	2.3
≥ 50 years	40	20	60	50	ref.	ref.
Total	100	100	200	100
ORs = OR with all surviving cases
ORtt = OR with all cases

FEM PAGE CONTRIBUTORS 2007

Contributor

Arnold Bosman