Operator bias refers to systematic differences in outcomes that arise due to variations in the skill, experience, decision-making, or preferences of the individual performing a procedure or intervention.

• Outcomes influenced by who performs the procedure, not just what is done
• Particularly relevant in surgical and interventional studies
• Often unacknowledged in retrospective analyses
• Can confound comparisons between techniques or centers

• A high-volume vascular neurosurgeon may achieve better outcomes with clipping than general neurosurgeons, skewing results in favor of surgery when comparing to endovascular treatment performed by less experienced interventionalists.

• Distorts the apparent efficacy or safety of a procedure
• Makes multicenter or multitechnique comparisons unreliable
• Introduces hidden bias in non-randomized studies

• Report operator volume and experience
• Perform stratified or sensitivity analyses by operator
• Acknowledge as a potential confounder in observational studies