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 [...]

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 [...]

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 [...]

Consider the figure below, which shows the KPI (key performance indicator) “inventory age in days” for 20 months. Looking at the chart raises a lot of questions. Why did inventory age shoot up by almost 40% between the 7th month and the 9th? Why did it suddenly drop down again? Why was there much more variability in [...]