".......The Institute for Healthcare Improvement's Global Trigger Tool found at least ten times more confirmed, serious events than these other methods. Overall, adverse events occurred in one-third of hospital admissions. Reliance on voluntary reporting and the Patient Safety Indicators could produce misleading conclusions about the current safety of care in the US health care system and misdirect efforts to improve patient safety."

Reference: Classen, D.C., et al., 'Global trigger tool' shows that adverse events in hospitals may be ten times greater than previously measured. Health Affairs, 2011. 30(4): p. 581-589.

http://content.healthaffairs.org/content/30/4/581.abstract