Learning curve deviation refers to an unexpected departure from the typical improvement trajectory expected as a surgeon gains experience with a procedure.

Instead of progressing steadily toward proficiency, a learner may show:

• Persistent high complication rates
• Recurrent technical errors
• Performance plateaus without improvement
• Sudden deterioration after initial improvement

CUSUM analysis is particularly effective at revealing these deviations in real time:

• A continuously rising CUSUM line suggests ongoing underperformance.
• A flat CUSUM line with no downward trend may indicate that proficiency has not been reached.
• An inverted learning curve (initial good results followed by worsening outcomes) can signal overconfidence or insufficient supervision.

• Inadequate case exposure or supervision
• Fatigue or cognitive overload
• Variability in case complexity
• Lack of structured feedback
• Poor ergonomics or equipment issues

When a learning curve deviation is detected:

• The trainee may benefit from additional mentorship or simulation.
• Case complexity can be temporarily reduced to consolidate skills.
• Objective review sessions using the CUSUM chart should be conducted.
• Training programs can adapt timing, expectations, or teaching strategies.

Learning curve deviations are normal, especially in technically demanding fields like neurosurgery. The key is early identification and targeted support—which is precisely what CUSUM facilitates.