By focusing solely on the output of the processes which produce the shafts and bearings, we are limited to making minor adjustments to the aim of one of these processes. This, in turn, results in only small improvements to the assembly process. What we need to do is change the focus of our efforts.

One of the fundamental goals of SPC is continual improvement through learning about your process. By using control charts to monitor the processes which produce the shafts and the bearings, and then taking the necessary action to remove the assignable causes, we can reduce the variation further upstream, which will have a tremendous impact on the assembly process. As we said earlier on, it can be possible to reduce the overall variation of a process by as much as 50% by bringing it into a state of statistical control. Let us take a look at what would happen if the variation in the shaft and bearing processes were to be reduced by 50%.