Defining and measuring patient safety.

Despite the growing demand for improved safety in health care, debate remains regarding the magnitude of the problem and the degree to which harm is preventable. To a great extent, this debate stems from variation in the definition and methods for measuring safety, its "shadow" error, and the degree of preventability. This article reviews the definition of safety and error, discusses approaches to measuring safety, and provides a framework for investigating incidents that unveils how the systems under which care is delivered may contribute to adverse incidents.