About the Process Reliability Line

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, equipment and location 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 and 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.

For example, in the following Production Analysis, notice the Process Reliability Line for the bottled water production process.

Reading the analysis from right to left, notice that the output closely follows the Demonstrated Line until the production output is around 3000 bottles per day. The output then decreases and begins to trend away from the Demonstrated Line toward a lower Reliability rate. We have drawn the Process Reliability Line where we observed this distinct change.

Notice that the line is labeled Process Reliability: 65.15%. This means that our process is reliable 65.15% of the time. The other 34.85% of the time, production loss is due to equipment/location failures, downtime, or intentional cutbacks in production.

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.

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