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rectly on the causes of adverse events, but attempts to classify adverse events according to "root causes" are complicated by the fact that several interlocking factors often contribute to an error or series of errors that in turn result in an adverse event.26,27 In recent years, some progress toward a more standardized nomenclature and taxonomy has been made in the medication area, but much work remains to be done.28

The following discussion of the literature addresses four questions:

1. How frequently do errors occur?

2. What factors contribute to errors?

3. What are the costs of errors?

4. Are public perceptions of safety in health care consistent with the evidence?

How Frequently do Errors Occur?

For the most part, studies that provide insight into the incidence and prevalence of errors fall into two categories:

1. General studies of patients experiencing adverse events. These are studies of adverse events in general, not studies limited to medication-related events. These studies are limited in number, but some represent large-scale, multi-institutional analyses. Virtually all studies in this category focus on hospitalized patients. With the exception of medication-related events discussed in the second category, little if any research has focused on errors or adverse events occurring outside of hospital settings, for example, in ambulatory care clinics, surgicenters, office practices, home health, or care administered by patients, their family, and friends at home.

2. Studies of patients experiencing medication-related errors. There is an abundance of studies that fall into this category. Although many focus on errors and adverse events associated with ordering and administering medication to hospitalized patients, some studies focus on patients in ambulatory settings.

Adverse Events

An adverse event is defined as an injury caused by medical management rather than by the underlying disease or condition of the patient.29 Not all, but a sizable proportion of adverse events are the result of errors. Numerous

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