Expected Outcome [Neurosurgery Wiki]

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===== Expected Outcome =====

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.

==== Clinical Definition ====

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

==== Role in CUSUM ====

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'')

==== Why It Matters ====

  * 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

==== Adjusting the Expected Outcome ====

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.