It has long been recognized that medical care has the potential to cause harm. However, general acknowledgment that much iatrogenic injury may be due to human error or system failures has been slower to emerge.

Every day, tens if not hundreds of thousands of errors occur in the U.S. health care system. Fortunately, most of these errors result not in serious harm but in near misses. A near miss is defined as an act of commission or omission that could have harmed the patient but did not do so as a result of chance (e.g., the patient received a contraindicated drug but did not experience an adverse drug reaction), prevention (e.g., a potentially lethal overdose was prescribed, but a nurse identified the error before administering the medication), or mitigation (e.g., a lethal drug overdose was administered but discovered early and countered with an antidote). Sadly, however, a small proportion of errors do result in adverse events—that is, they cause harm to patients—exacting a sizable toll in terms of injury, disability, and death.

To Err Is Human estimates that 44,000 to 98,000 hospitalized patients die annually in the United States and that more than 1 million patients are injured as a result of error. These estimates were based on the findings of studies conducted in Colorado and Utah (Thomas et al., 1999, 2000) and on the Harvard Medical Practice Study conducted in New York State (Leape et al., 1991), extrapolated to hospital admissions throughout the nation. These epidemiological studies helped jump-start a process to define the overall scope of the safety problem. Elsewhere, the Quality in Australian Health Care Study (Wilson et al., 1995) generated similar findings for hospital-based care, as did a preliminary study in the United Kingdom (Vincent et al., 2001).

An error is the failure of a planned action to be completed as intended (i.e., error of execution) or the use of a wrong plan to achieve an aim (i.e., error of planning). An error may be act of commission or an act of omission.

It is not possible to quantify the full magnitude of the safety challenge with certainty. The health care sector does not routinely identify and collect information on errors. Experts have challenged the estimates of patient harm attributable to errors as well as the methodologies used to derive them (Brennan, 2000; Hayward and Hofer, 2001; Sox and Woloshin, 2000; McDonald et al., 2000). As substantial evidence about adverse events continues to accumulate in the United States and other countries, however, there

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