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