Why: Failure rates are different for both people and equipment at different phases of operation and the medicine to be applied to both humans and equipment need to be considered for effectively treating the roots of the problem.
When: The concept is useful during design, operation, and maintenance of equipment and systems to understand the failure mechanisms
Where: It explains the human experiences to the ordinary person to relate equipment/system failures to those experienced in real life so as to coordinate the design, operation and maintenance of equipment. For other definitions see MIL-HDBK-338, section 9.
These definitions are written by H. Paul Barringer and are also posted on his web site at www.barringer1.com