===== 🕰️ Chronological Bias =====

**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.

==== ⚠️ Key Characteristics ====
  * 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  

==== 🧪 Example in Neurovascular Studies ====
  * 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.

==== 📉 Why It Matters ====
  * 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  

==== ✅ Best Practice ====
  * Use **contemporaneous control groups**  
  * Include **time as a covariate** in statistical models  
  * Acknowledge in the study limitations and avoid overinterpreting results across time spans