The historical complication rate is the average rate of adverse outcomes observed in a specific procedure over a defined period. It serves as the foundation for setting the reference value (k) and the CUSUM threshold in surgical performance monitoring.

In CUSUM analysis, this historical rate provides a realistic performance benchmark, allowing current outcomes to be compared meaningfully with past institutional or published data.

To determine the historical complication rate for intracranial pressure (ICP) monitor placement:

• Review all relevant cases over the past 1–3 years.
• Count the total number of complications (e.g., infection, hemorrhage, malposition).
• Divide by the total number of procedures performed.

Example:

• 12 complications over 120 ICP placements
• Historical complication rate = 12 / 120 = 10% → k = 0.1

• Establishes the target used in the CUSUM equation (reference value k).
• Serves as a baseline to detect improvements or deteriorations in current performance.
• Helps differentiate between expected risk and performance-related issues.

• Institutional surgical audit records
• National registries or multicenter studies
• Peer-reviewed literature (e.g., complication rates from similar patient populations)

• Use local data when available for the most accurate benchmark.
• Update the historical rate periodically to reflect improvements in technique or changes in patient population.
• Stratify data by factors like procedure type, surgeon experience, or patient complexity when possible.

By grounding your CUSUM analysis in real-world historical data, you ensure that the performance thresholds are both clinically meaningful and fairly applied.