Leape discusses how traditional methods of error reduction in medicine have focused on individual performance rather than on the systems in which individuals operate. With reference to Reason, he briefly reviews the cognitive psychology of human error, distinguishing between performance and error at the schematic or "skill" level, where an error is a "slip" (or "lapse"), and at the rule-based or "knowledge" level, where an error is a "mistake." Using the aviation industry as an example, Leape advances a systems-based approach to improving patient safety. Rather than relying on the absence of human error, as has been traditional in medicine, he advocates systems that assume human errors will occur and that are designed to minimize their occurrence and absorb them when they happen. He reviews several specific systems modifications to accomplish this transformation, as well as advocates national policy changes to institutionalize safety improvement.