Why: The metric provides an awareness factor for deciding central tendency numbers and for the expected number of events which will occur into the future based on historical situations. The arithmetic simplicity of mean time is a reason to establish the metric, and listen to the information derived from it to gain insight. The arithmetic provides immediate answers to categorize facts for starting continuous improvement rather than postponing a metric while searching for delayed perfection!
When: The metrics are used as criteria of performance and variations from the central tendency numbers are expected however for the long term the variations are expected to be controlled to prevent distortion of the measurement.
Where: The metrics are use from the shop floor to the management levels as criteria for "How are we doing?".
These definitions are written by H. Paul Barringer and are also posted on his web site at www.barringer1.com