If an under construction driving highway is assumed as process to be worked on then the vehicles available on the road can be taken as variability of the process. The specification limits will be the lines defining the driving lanes.
From Monday to Friday, you take different vehicles on the same route and had different experiences with each vehicle. Depending on the size of each vehicle you had to be careful about the lanes, their size, how to drive in those lanes and the possible damage that could be caused to the vehicle.
Through this activity, you experienced varying degrees of process capability, ranging from good process capability to poor process capability. All the vehicles are running straight down through the lane the process is centered on nominal, which is equivalent to Cp, the process potential. This Cp is calculated as the width of the specification, which was the vehicle, divided by the variability.
The vehicle or the process was being run off center, thereby getting closer to the barrier on one side. As you approach the barrier, the opportunity for success is lowered. This is the Cpk process performance, the distance from the center of the vehicle to the nearest barrier, which is divided by the distance from the center of the vehicle to the edge of the vehicle closest to that same barrier. The primary reason of using the nearest side is that it is the only one which has danger of being damaged and affecting the Cpk. The Cpk of the process; therefore, needs to be taken care of.
If the process has a Cp of less than 1, even if the process is centered, the result would be a bad product. Therefore, there are two options:
- Broaden the specification limits.
- Reduce the variation so that the process fits between the existing barriers.