Interpretation & Calculations
The following is an excerpt from Chapter 11 of Pyzdek Guide to SPC Volume II by Thomas Pyzdek, © QA Publishing, LLC.
Starting and administering SPC is not a trivial task. Many traps and pitfalls await the unwary. If not handled properly, a number of unproductive situations can result from the attempt, such as
1. Beauty contest programs. These are massive displays of control charts, histograms, cause and effect diagrams etc. which serve no useful purpose. These displays are often placed in "war rooms" which are located far ways from the processed being monitored. The charts are usually computer generated and the charts are often very colorful and of near-typeset quality. The problem is, the people who can use them either do not ever see them, or see them too late for the result to serve any useful purpose.
2. My little darling programs. These are programs started by an individual or small group, usually in the quality department, with little or no active management leadership. Those who start these programs have often recently attended a seminar and learned about the statistical aspects of SPC, but not the management or human relations aspects. Charting and data collection is started before creating a management system and environment for dealing with the problems highlighted by SPC.
3. "The greatest show on earth!" These programs are usually launched by training or human resources departments and they usually feature speeches by senior management, suggestion programs (almost always called something cute), buttons, badges, hats, flags and other paraphernalia, etc. Everyone is told the obvious ("quality is important to our customer, and to us!") and asked to give 110% to the cause of quality without ever being told exactly what that means. SPC is implemented on a hit-and-miss basis and everyone is expected to understand SPC with little training or guidance. These programs never die, rather, like old soldiers, they simply fade away.
4. Drill instructor programs. These programs are started by autocratic upper managers who have, by god, had it with the rubbish being produced by the "troops." You will implement SPC, you will produce quality! Of course, once the proclamation has been made, the leader retires from the scene to let someone else handle all of the details. When the smoke clears and no progress has been made, heads may roll. But effective quality improvement is highly improbable.
5. Potpourri. This sort of SPC consists of a melange of statistical and pseudostatistical tools splattered about. Over here we have a "control chart" with control limits that are really the engineering specifications. Over there is a control limits that are really the engineering specifications. Over there is a chart with no limits of any kind, put in place to "help people get used to the idea of plotting data." Somewhere else, Mil-Std-105E is being applied and the results plotted on a p-chart. Such an approach results in total chaos, but management can be easily duped into thinking that they are witnessing a truly sophisticated rendition of SPC.
6. The island of excellence. This is the immortal pilot SPC project. It was originally started to "get a feel for how SPC can be used here at XYZ Corp.," but expansion never quite got off of the ground. Nevertheless, management was quick to see the PR value of the effort and the refused to let it die. The main purpose becomes eyewash for guests, especially customers who want to see tangible evidence that you are serious about quality improvement.
The list can be extended ad nauseam, but you get the idea. The important lesson here is that successful SPC is no accident. To assure success SPC must be carefully planned and it must receive the active attention of every level of management, from top management on down.
Learn more about the SPC principles and tools for process improvement in Statistical Process Control Demystified (2011, McGraw-Hill) by Paul Keller, in his online SPC Concepts short course (only $39), or his online SPC certification course ($350) or online Green Belt certification course ($499).