They define what the process will deliver as long as it continues to operate as consistently as possible. When a process displays a reasonable degree of statistical control, it is operating as consistently as possible. The process doesn't really care whether you like the natural process limits, and it certainly does not know what the specifications may be. Specifications should be thought of as the voice of the customer, which is distinctly different from the voice of the process. Therefore, if you are not pleased with the amount of variation shown by the natural process limits, then you must go to work on the system to change the underlying process rather than setting arbitrary goals, asking for reports, jawboning the workers, or looking for alternate ways to compute the limits.
Mere description encourages inappropriate actions.
It makes routine variation look like signals that need attention. In this case there were no signals in the data, yet none of the traditional ways of looking at the data revealed this absence of signals. Analysis discourages inappropriate actions by filtering out the noise before any potential signals are identified. The difference is profound. Select another article.