Figure 4.

Directed acyclic graph illustrating selection bias by health-care-seeking behavior. A) Health-care-seeking HS confounds the relationship between vaccination V and influenza status I and also influences testing and selection into the study T. Influenza status may also influence a person to seek care and be tested. Additional confounders of the V→I relationship are represented by C and may also influence HS. Only patients who are tested for influenza (T = 1) are included in the study, resulting in collider bias. B) Control of HS (HS = 1) blocks the biasing path.