Lukas Murdock
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Statistical Process Control

Planted 02023-08-08

Statistical process control is the application of statistical methods to monitor and control the quality of a production process. Statistical process control is appropriate to support any repetitive process, including financial auditing and accounting, IT operations, health care processes, and clerical processes such as loan arrangement and administration, customer billing etc.

ISO 9000

Walter A. Shewhart pioneered SPC at Bell Laboratories in the early 1920s, developed the control chart in 1924.

W. Edwards Deming invited Shewhart to speak at the Graduate School of the U.S. Department of Agriculture and served as the editor of Shewhart’s book Statistical Method from the Viewpoint of Quality Control (1939), which was the result of that lecture.

Process Behavior Charts (Shewhart individuals control chart)

Use

  • Find out when a change has occurred in a process
  • Evaluate if your process changes made any significant difference to the process
  • Determine whether a process is predictable or not
  • In predictable processes, predict the range within further measurements will fall if the conditions do not change
  • Assess a process performance against a requirement e.g. customer demand and expectations.

References

Calculations

  • Some value with ideally over 23 data points
  • calculate average(X)
  • calculate mR
  • calculate average(mR)
  • calculate UNPL = average(x) + (2.66 * average(mR))
  • calculate LNPL = average(x) - (2.66 * average(mR))
  • calculate URL = 3.269 * average(mR)

Terms

  • mR – moving range, the difference between two successive values
  • UNPL – Upper Natural Process Limit, gives us the best case scenario forecast
  • LNPL – Lower Natural Process Limit, gives us the worst case scenario forecast
  • URL – Upper Range Limit

Interpreting

  1. Any point outside the limits indicates the presence of an exceptional cause that has a dominant effect.
    • If not accounted by cyclical or seasonal patterns
  2. Run of eight successive values all on the same side of the central line may indicate the presence of an exceptional cause that has a weak but sustained effect.
  3. Runs near the limits three successive values all within 25% of the upper or lower limits may indicate the presence of an exceptional cause that has a moderate but sustained effect.
  4. Alternating at short intervals likely means two systems of causes that need to be separated.
  5. Wide limits are a noise warning that it would be a mistake to react to changes in these values as it is virtually impossible to assign an explanation to why a point goes up or why it goes down.

In a stable process, most of values fall within the 50% area between the limits.

Deming’s 14 points for management

Making Sense of Deming

Dr. W. Edwards Deming offered 14 key principles for management to follow to improve the effectiveness of a business or organization significantly. The principles (points) were first presented in his book Out of the Crisis. Below is the condensation of the 14 Points for Management, but these alone will not improve your business.

Dr. Deming’s 14 Points for Management

  1. Create constancy of purpose toward improvement of product and service, with the aim to become competitive and to stay in business, and to provide jobs.
  2. Adopt the new philosophy. We are in a new economic age. Western management must awaken to the challenge, must learn their responsibilities, and take on leadership for change.
  3. Cease dependence on inspection to achieve quality. Eliminate the need for inspection on a mass basis by building quality into the product in the first place.
  4. End the practice of awarding business on the basis of price tag. Instead, minimize total cost. Move toward a single supplier for any one item, on a long-term relationship of loyalty and trust.
  5. Improve constantly and forever the system of production and service, to improve quality and productivity, and thus constantly decrease costs.
  6. Institute training on the job.
  7. Institute leadership. The aim of supervision should be to help people and machines and gadgets to do a better job. Supervision of management is in need of overhaul, as well as supervision of production workers.
  8. Drive out fear, so that everyone may work effectively for the company.
  9. Break down barriers between departments. People in research, design, sales, and production must work as a team, to foresee problems of production and in use that may be encountered with the product or service.
  10. Eliminate slogans, exhortations, and targets for the work force asking for zero defects and new levels of productivity. Such exhortations only create adversarial relationships, as the bulk of the causes of low quality and low productivity belong to the system and thus lie beyond the power of the work force.
  11. (a) Eliminate work standards (quotas) on the factory floor. Substitute leadership. (b) Eliminate management by objective. Eliminate management by numbers, numerical goals. Substitute leadership.
  12. (a) Remove barriers that rob the hourly worker of his right to pride of workmanship. The responsibility of supervisors must be changed from sheer numbers to quality. (b) Remove barriers that rob people in management and in engineering of their right to pride of workmanship. This means, inter alia, abolishment of the annual or merit rating and of management by objective.
  13. Institute a vigorous program of education and self-improvement.
  14. Put everybody in the company to work to accomplish the transformation. The transformation is everybody’s job.