Archive for the 'Process control' Category

How Western Electric rules mislead in statistical process control

The statistical model behind control charts for in-control processes is based on the assumption a Gaussian process with no autocorrelation (i.e. independent) with a constant mean and constant variance: in other words a white noise process. The various Western Electric rules try to find patterns that are not white noise, and thus show that the [...]

Investing in measurement usually pays off

A headline in British journal “The Engineer” caught my eye a few weeks ago. It said that the British government has announced a £240 million (US $393 million) investment in measurement, with the aim of improving measurement techniques and technology to stimulate innovation in productions, processes and services. The heart of the scientific process is [...]

More circular reasoning on statistical process control

After my last post I received a number of comments on the website KPIExperts.  Most of them completely misunderstood my point, and their misunderstanding was so fundamental that rather than reply to their comments individually I decided to write a new post.  I myself have trained thousands of people in SPC over more than two [...]

Circular Reasoning in Statistical Process Control

In a Statistical Process Control (SPC) chart, measurements are plotted on a chart with upper and lower “control limits.” The idea is to compare the plotted points with the limits to see if a process is stable, and to identify “special” causes of variation.  The control limits are supposed to indicate when action should be taken [...]