What is the whole point of making control charts?
What action is it that both Shewhart and Deming where trying to get people to avoid?
Statistical thinking is based on these three concepts: [Foster 2010]
All occur in a system of interconnected processes.
All processes have variation, random and non-random.
Understanding the latter (non-random) and reducing it are critical.
Control charts are developed with statistical thinking in mind, which guides us to make decisions based on the analysis of data, and more precisely for monitoring process variation. The typical chart is shown in Figure 1; it has an upper limit, a center line, and a lower limit.
Walter Shewhart and W. Edwards Deming crafted the foundation for statistical quality control, but their work was part of an evolution that started back then in the 1920’s, when the former invented the concept of control charts while working for Bell Laboratories, and H.F. Dodge and H.G. Romig developed acceptance-sampling methods. By that time, United States was enjoying postwar prosperity and quality methods started to arise as they provided a competitive edge in mass production, which allowed the price of goods such as cars to go down.
Fig 1 - Control Chart
Figure 2a shows the migration of manufacturing from a craft to process control, while craftsmen’s process looks more like a service in which the customer participates throughout – from specifying to verifying the custom product. Frederick Taylor separated the craftmen’s work into specialized series of tasks, removing the customer from several tasks. Then sampling plans were developed and Shewhart experimented to observe patterns in the process output and developed statistical methods to characterize the product output. Figure 2b illustrates how statistical methods – so called control charts – were developed to control the process. Shewhart’s approach appears to be a method to manage the product output through process control using his out of control action plans. The Shewhart cycle of process control was revisited by W. Edwards Deming, and eventually named the plan-do-check-act (PDCA) cycle as shown in Figure 2c. [Quality Progress 2006]
Fig. 2 - PDCA's Beginnings
Deming’s revised version of process control was a means to reduce variation between the desired and actual performance and the key element is quality at the source -Deming’s 14 Point -#3 Cease mass inspection to improve quality. [Foster 2010]
Concluding, it can be said that both, Shewhart and Deming, wanted the people (workers) to avoid passing defective units to the next process by making all workers responsible for their own work and perform needed inspections at each stage of the process to maintain process control.
References
Foster, Thomas S. Managing Quality – Integrating the Supply Chain. Prentice Hall 4th Edition. 2010. Print.
Quality Progress. PDCA’s Beginnings. 39 no 7 JI 2006. P.47.
