Yeah, that is actually a slight underestimate. Basically the average hospital patient receives 120 procedures per day, with a 1% error rate. Of those errors, an estimated 10 - 15% cause death or disability. The links I posted in my other comment to that IOM article and the Leape analysis of it basically support this, although I can't find an exact cite for that 120 procedures per day stat.

edit: It looks like I'm mixing measurements slightly. Basically I did find the source of that stat, it is here:

"There was an average of 178 activities per patient per day and an estimated number of 1.7 errors per patient per day. For the ICU as a whole, a severe or potentially detrimental error occurred on the average twice a day. Physicians and nurses were about equal contributors to the number of errors, although nurses had many more activities per day."

http://www.ncbi.nlm.nih.gov/pubmed/7867355

However, this is from the IOM report:

"Two large studies, one conducted in Colorado and Utah and the other in New York, found that adverse events occurred in 2.9 and 3.7 percent of hospitalizations, respectively. In Colorado and Utah hospitals, 6.6 percent of adverse events led to death, as compared with 13.6 percent in New York hospitals. In both of these studies, over half of these adverse events resulted from medical errors and could have been prevented.

When extrapolated to the over 33.6 million admissions to U.S. hospitals in 1997, the results of the study in Colorado and Utah imply that at least 44,000 Americans die each year as a result of medical errors. The results of the New York Study suggest the number may be as high as 98,000."

Also from the IOM report:

"In a study of 1,047 patients admitted to two intensive care units and one surgical unit at a large teaching hospital, 480 (45.8 percent) were identified as having had an adverse event, where adverse event was defined as “situations in which an inappropriate decision was made when, at the time, an appropriate alternative could have been chosen.” 41 For 185 patients (17.7 percent), the adverse event was serious, producing disability or death. The likelihood of experiencing an adverse event increased about six percent for each day of hospital stay."

Then Leape in his own analysis explains why even the 98,000 figure is probably a fairly large underestimate:

"The [IOM] study’s most serious limitation is probably that it was a retrospective medical record review study. Many important events in patient care are not recorded in the medical record. Some errors are not even known to clinicians caring for the patient. Studies of autopsy, for example, have found potentially fatal misdiagnoses in 20% to 40% of cases. On balance, the reliance on information extracted from medical records most likely led to a substantial underestimate of the prevalence of injury."

"They undoubtedly missed some that did occur because many adverse events and errors are never recorded in the medical record, either because they are concealed or not recognized. Other errors are discovered after the patient is discharged. In fact, in the MPS, an additional 6% of hospital-caused adverse events were discovered after discharge, but were excluded from the analyses because they were an unknown fraction of all such events. Therefore, any record-review study produces at best a 'lower bound.'"

"Second, neither of the large studies examined the extent of injuries that occur outside of the hospital. More than half of surgical procedures (numbering now in the tens of millions) take place outside of a hospital setting, and the adverse event rates for these procedures have not been studied. Even if complication and death rates are much lower than in-hospital care, the absolute numbers must be substantial, as suggested by the recent report of deaths associated with liposuction."

"Third, when prospective detailed studies are performed, error and injury rates are almost invariably much higher than indicated by the large record-review studies. In a large study of patients who died from acute myocardial infarction, pneumonia, or cerebrovascular accident (conditions that account for 36% of all hospital deaths), DuBois and Brook found that 14% to 27% of deaths were preventable. Andrews et al found that 17% of intensive care unit patients had preventable serious or fatal adverse events. The Centers for Disease Control and Prevention estimates that 500,000 surgical-site infections occur each year. One large controlled study found the excess mortality rate of surgical-site infections to be 4.3%, suggesting 20,000 deaths annually from this cause alone. These data are strong evidence that record-review studies seriously underestimate the extent of medical injury."