An ideal process will produce the maximum possible output at all times, but actual processes demonstrate variation in output over time. The variation in output can be caused by various factors, including process problems, Asset problems, shutdowns, and cutbacks.
The Process Reliability Line marks the location at which datapoints begin to trend away from the Demonstrated Line. This is the location at which variation in output shifts from being attributed to production losses to being attributed to reliability losses. It defines the reliability of the process, expressed as a percentage. It is the point at which the process shifts from being fairly reliable to being unreliable.
The decision of where to draw the Process Reliability Line is somewhat subjective and will be determined by how your datapoints look after they have been plotted and how you interpret the plot. If your datapoints do not exhibit a distinct trend away from the Demonstrated Line, and are plotted in a fairly, straight, diagonal line, it may not be appropriate to draw the Process Reliability Line at all.
The following image shows an example of a Process Reliability line showing that reliability of the process is 56.65%:
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