Archive for the 'Identifying changes in indicators' 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 [...]

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 [...]