The Use of the Variogram to Elaborate Meaningful Process Control Charts

Forty years ago Dr. Pierre M. Gy, a metallurgist, introduced variography as applied to monitoring a process. Consequently, his followers used the variogram to elaborate meaningful process control charts, especially since 1993. The name “chronostatistics” was suggested several times by Gy as a logical name for a new way to interpret any data collected in a chronological way: the intent was to draw a parallel between the variogram identifying the variability components affecting a process parameter as a function of time, and the variogram used in geostatistics identifying the variability components of a given mineral content as a function of space. Heterogeneity problems encountered indeed have strong similarities. However, there is a fundamental difference: the variogram used in geostatistics is more likely to be reasonably stationary within a given type of mineralization. While investigating a dynamic process through chronostatistics, there is no such thing as a stationary variogram. Still, the evolving variogram provides essential information about subtle, often overlooked process difficulties. Chronostatistics is especially useful for management to take advantage of existing data that are often a forgotten source of valuable information to pinpoint recurring structural problems. The superiority of chronostatistics over conventional statistical process control approaches is that it links observations to the inherent heterogeneity of materials under investigation, therefore to the Theory of Sampling. Chronostatistics is like opening a new world to the metallurgist, making the process variability speak in a more comprehensive and in-depth way. Several case studies are presented to demonstrate the potency of this new science: for each of them, observations lead to solutions for continuous process improvement, therefore attractive economic opportunities. The philosophy is this: if a control chart shows a process is out of control, it must also suggest logical and pragmatic directions for solutions. This chapter intends to vastly expand on these valuable, almost forgotten, ideas.