Quality & Business Improvement

Bluffer's Guide to Statistical Process Control

General notes

Advantages of SPC

Disadvantages of SPC

Structure of control charts

The nominal (centre line) is dotted; above and below are two solid lines representing the Upper Control Limit and the Lower Control Limit, each of which is three standard deviations from the centre line.

The limitations are that 15-25 data points are needed before the limits can be calculated and that only one characteristic can be tracked per control chart

Even if part characteristics are similar, other differences such as material type or specification limits may mean that separate control charts will have to be constructed.


SPC can only operate satisfactorily in a company if:

One way to consider SPC is to realise that while all data contains noise, only some data contain signals. If there is no way to separate the probable noise from the potential signals, then there is the probability of being misled by the noise in the data. Shewhart's Control Charts are the simplest way to separate signals from noise.

Further Reading

  Introduction to Statistical Quality Control
by Douglas Montgomery

ISBN: 0471316482, Hardcover - £86.50 BUY
ISBN: 047121311X, Paperback - £xx.xx BUY

This is the standard text on Statistical Process Control, a smooth, timeless classic, which should be on every quality engineer's bookshelf. It's certainly on mine. The first chapter represents Montgomery's own personal philosophy and musings, with some parts completely unchanged from previous editions, but after that, it's all business. Don't be fooled by the "introduction" part of the title. It's all good, solid statistics.

  SPC in the Office
Mal Owen, John Morgan

ISBN: 0952332841, Hardcover - £40.00 BUY

SPC and Continuous Improvement
Mal Owen

ISBN: 0984507957, Hardcover - £45.00 BUY

Mal Owen taught me SPC: his courses were so popular, there was practically a stand-up fight among people wanting to attend. These books, I think, are much easier to understand than Montgomery (not necessarily better, of course) and Owen deserves special credit for bringing SPC out of its traditional manufacturing territory into the paper-filled jungle of the office.

© Fell Services Ltd., 2004