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 [...]
How statistical process control charts mislead
The statistical process control chart is designed to help differentiate between what are usually called “common” causes of variation and “special” causes. But this is very misleading. Typically, six-sigma consultants and various authors advise you to ignore data between control limits because it is “random.” They say to only make process adjustments when the process [...]
Throwing out water quality data
With real time measurements, exception reports are often the most useful indicator. This seems to have been forgotten by one government water quality inspector.
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