What is SPC?

The aim of Statistical Process Control (SPC) is to establish a controlled process by the use of statistical techniques to reduce process variation. A decrease in variation will lead to:
  • better quality;
  • lower costs (waste, scrap, rework, claims, etc.);
  • more insight into the capability of the process.
To arrive at a controlled process (see graph below):
  • the measurements have to be registered in a correct way (MSA);
  • the data has to be analyzed in a correct way (SPC);
  • decisions must be taken based on the analysis and a proper procedure (OCAP);
  • process adjustments must be registered to follow up and evaluate the effect of adjustments (PDCA).


In typical manufacturing methods, measurements are compared to specification limits and the result is a pass/fail decision. There is no indication of process variation or indeed if there are any disturbances (relative to a controlled process). It is important to establish a normal variation pattern for the process and maintain it by continual process monitoring. If there is a deviation from a normal variation then a disturbance has occurred and process adjustments have to be made. SPC supplies the techniques for measuring, recording, analyzing and decision making. When all disturbances or special causes of variation are eliminated the process is said to be in statistical control. However, SPC is much more than just analyzing technical processes, it’s not just the implementation of control charts it must be seen as or must be used in combination with, a total quality program.


In spite of SPC coming to the fore in western industries during the 1980’s, the origins of SPC lay in the twenties in America. Walter Shewhart, Bell Telephone Laboratories, USA in 1924 developed statistical quality control methods for the improvement of product quality. These methods were incorporated into a management philosophy by Dr. W.E. Deming (a younger colleague of Shewhart). Just prior to World War II American industrial management did not pay very much attention to Deming and his views on statistical techniques and open management style. However, Japan’s post-war efforts to increase production, and to compete with western industries, found Deming’s philosophy attractive. Top Japanese management concluded that they had to improve quality, and invited Deming to lecture in Japan in the early 1950’s. The successful tour, lead to a few companies implementing the Deming methodologies and within a few month's their quality and productivity increased. This, in turn, led to a greater proliferation of these techniques in Japan. It was the commitment of top Japanese management, the realization of the rewards of SPC implementation plus the philosophies of Deming that are the basis of Japanese competitive advantage as we know it today. Deming stated that a quality product can only be made if all the processes in a company are under control, therefore everybody in a company is responsible for quality. The knowledge on the shop floor has to be used and the walls between departments have to be torn down. It is the responsibility of management to allow operators to work with the best methods, the best machines, etc. In 1981 Deming appeared in a documentary on American television named: “If Japan can, why can’t we?” There was a considerable reaction and for the first time managers in America listened to his philosophy. It was quickly proven that SPC could also give beneficial results in western industries. However, despite increased attention on this side of the globe; SPC is still in a preliminary implementation stage. Deming has summarized his philosophy in 14 rules of management, these are given below.
  • Create constancy of purpose toward improvement of product and service with the aim to become competitive, stay in business and provide jobs.
  • Adopt the new philosophy. We are in a new economic age, created by Japan. A transformation of the Western style of management is necessary to halt the continued decline of the industry.
  • Cease dependence on inspection to achieve quality. Eliminate the need for mass inspection by building quality into the product in the first place.
  • End the practice of awarding business on the basis of price. Purchasing, design, manufacturing and sales departments must work with the chosen suppliers so as to minimize total cost, not initial cost.
  • Improve constantly, and forever, every activity in the company so as to improve quality and productivity and thus constantly decrease costs.
  • Institute education and training on the job, including management.
  • Institute improved supervision. The aim of supervision should be to help people and machines do a better job.
  • Drive out fear so that everyone may work effectively for the company.
  • Break down the barriers between departments. People in research, design, sales and production must work as a team to tackle production and usage problems that may be encountered with the product or service.
  • Eliminate slogans, exhortations and targets for the workforce asking for new levels of productivity and zero defects. The bulk of the causes of low quality and low productivity belong to the system and will not be in the direct control of the workforce.
  • Eliminate work standards that prescribe numerical quotas. Instead, use resources and supportive supervision, using the methods to be described for the job.
  • Remove the barriers that rob the hourly worker of the right to pride of workmanship. The responsibility of supervision must be changed from sheer numbers to quality. Equally, remove barriers that rob people in management and engineering of their right to pride of workmanship.
  • Institute a vigorous program of education and retraining. New skills are required for changes in techniques, materials, and services.
  • Put everybody in the organization to work in teams to accomplish the transformation.

Rules of Deming


Traditionally manufacturers have accepted that every product has to have tolerance limits because it is impossible to manufacture products without variation. Tolerance limits have been used historically as the basis for quality control i.e. product test results are either inside or outside of them. Intermediates and end-products are therefore inspected after manufacture and either pass or fail. With mass production, every product is checked and people use sampling plans to check if a lot can be shipped to the customer. The problem with these methods is that variation is not taken into account and in case of inspection in the warehouse that inspection is not immediately after a product is produced so people are not able to correct problems instantly and are also not able to find the root cause because the people doing the inspection are normally not the people in production. SPC aims to eliminate all disturbances in a process, reduce variation, and produce on target, thus leading to continual process improvement. Errors in the process such as tool wear, wrong adjustments, wrong materials etc. will be found at an early stage thus enabling production with less variation and reduced scrap levels.


The overall objective of SPC is to continuously improve processes by reducing variation. Meeting the overall objective makes it possible to meet other related objectives, like:
  • increase customer satisfaction and fewer customer complaints;
  • reduce or eliminate the need for inspection in the supply chain;
  • establish a predictable and consistent level of quality;
  • decrease scrap, rework and inspection cost;
  • increase the motivation of operators;
  • increase productivity;
  • lower investment because of process improvements;
For more reading look at www.statisticalprocesscontrol.com

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