Why: Maintainability measures the percent of maintenance jobs completed to a standard time for the repair with repair times for the task usually plotted on a lognormal probability plot.
When: First you set a standard repair time for the task, second you set a skills level, third you measure how you're doing against the standard.
Where: Applies to major tasks where many repetitions are expected and where considerable time is required.
These definitions are written by H. Paul Barringer and are also posted on his web site at www.barringer1.com