The expected outcome refers to the anticipated result of a surgical procedure under normal conditions, based on historical data, clinical standards, or institutional experience.

In CUSUM analysis, the expected outcome is used to establish a reference value (k)—typically the acceptable complication or failure rate. Each individual case is then compared to this expected performance level.

For intracranial pressure (ICP) monitor placement, an expected outcome is:

• Correct placement
• Functional monitoring system
• No complications such as hemorrhage, infection, or misplacement

If the acceptable complication rate is 10%, then the expected outcome rate is 90%, and:

• k = 0.10 → expected failure rate

Each case contributes positively or negatively to the cumulative sum, depending on whether the actual outcome matches the expected outcome:

• Success (0) → better than expected

→ CUSUM decreases slightly (e.g., Cₙ = Cₙ₋₁ - 0.1)

* **Complication (1)** → worse than expected  
  → CUSUM increases (e.g., ''Cₙ = Cₙ₋₁ + 0.9'')

• Sets a clear performance benchmark
• Defines what is considered normal vs. concerning
• Anchors the CUSUM curve in evidence-based practice
• Prevents overreaction to single adverse events when outcomes are still within expected variation

Expected outcomes may vary depending on:

• Patient risk profile
• Surgeon experience
• Case complexity
• Institutional or national guidelines

Therefore, the expected outcome must be explicitly defined and periodically reviewed to ensure meaningful performance monitoring.