Process A

Process A is a stable process. There is nothing significantly different about any of the individual vehicles. The approach to making sure all the results are in the OK zone is to do something to improve all the results. For example, improve the average as shown below:

Or make all the results closer to the average, which will appear as less horizontal spread on the histogram:

The important point about improving the results of a stable process is that you should investigate the things in the process which affect every result.

Process B

Process B is an unstable process. It has three distinct groups of vehicles and the performance of each group is significantly different. The approach to getting all the results into the OK zone with this process is to identify the vehicles which belong to the slow group, and stop using them.

Although all the results are now in the OK zone this is still an unstable process. We should determine which of the two remaining vechicle types gives the better balance between cost and performance, then use only that type.

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