The threshold value in CUSUM analysis defines the alert level at which the cumulative sum of deviations signals a statistically significant change in performance.

It serves as a boundary for action:

• When the CUSUM curve crosses the threshold, it may indicate deteriorating performance (e.g., rising complication rate).
• Staying below the threshold suggests performance remains within acceptable limits.

Threshold values can be:

• Statistical, based on type I/II error probabilities and decision intervals (commonly used in industrial applications).
• Empirical, based on institutional experience or expert consensus.
• Graphical, set arbitrarily for visual feedback (e.g., 2.5 in our ICP example).

In our analysis of ICP monitor placement, we used a threshold of 2.5 to flag deviations from the acceptable complication rate of 10%. This value is not fixed and can be adjusted depending on:

• Procedure complexity
• Acceptable risk tolerance
• Training level of the surgeon

Crossing the threshold may prompt:

• Case review or morbidity and mortality (M&M) analysis
• Temporary increase in supervision or training
• Modification of technique, workflow, or equipment

Using CUSUM with a clearly defined threshold value transforms raw outcome data into a real-time decision-support tool for improving patient safety and clinical performance.

Institutions may modify the threshold value based on:

• Historical complication rates
• Supervision level
• Case complexity
• Acceptable risk policy