Chronological bias occurs when changes over time—such as improvements in technique, technology, protocols, or team experience—affect the outcomes of an intervention, creating a biased comparison between groups treated at different time periods.

• Treatment groups span different eras in clinical practice
• Advances in perioperative care, imaging, or equipment are not controlled
• Often present in retrospective cohort studies over long periods
• Can confound outcomes, attributing differences to the treatment rather than the time in which it was applied

• Comparing aneurysm clipping from 2012–2015 with coiling from 2018–2022 introduces chronological bias if the latter benefits from updated antiplatelet protocols, improved coils, or better ICU management.

• Threatens the internal validity of comparative studies
• Masks or exaggerates true differences between interventions
• Creates false trends in outcomes attributed to technique rather than time

• Use contemporaneous control groups
• Include time as a covariate in statistical models
• Acknowledge in the study limitations and avoid overinterpreting results across time spans