Statistical Process Control – SPC

What is Statistical Process Control?

Statistical process control (SPC) is defined as the use of statistical techniques to control a process or production method. SPC tools and procedures can help you monitor process behavior, discover issues in internal systems, and find solutions for production issues. Statistical process control is often used interchangeably with statistical quality control (SQC).

Statistical Process Control (SPC) is an industry-standard methodology for measuring and controlling quality during the manufacturing process. Quality data in the form of Product or Process measurements are obtained in real-time during manufacturing. This data is then plotted on a graph with pre-determined control limits. Control limits are determined by the capability of the process, whereas specification limits are determined by the client’s needs.

SPC Tools

A popular SPC tool is the control chart, originally developed by Walter Shewhart in the early 1920s. A control chart helps one record data and lets you see when an unusual event, such as a very high or low observation compared with “typical” process performance, occurs.

Control charts attempt to distinguish between two types of process variation:

  • Common cause variation, which is intrinsic to the process and will always be present
  • Special cause variation, which stems from external sources and indicates that the process is out of statistical control

The preparatory phases of SPC involve several steps using a number of different tools.

Eight quality tools are available to help organizations to better understand and improve their processes. The essential tools for the discovery process are:

  • Check Sheet
  • Cause-and-Effect Sheet
  • Flow Chart
  • Pareto Chart
  • Scatter Diagram
  • Probability Plot
  • Histogram
  • Control Charts
  • Brainstorming

Why Use Statistical Process Control (SPC)

Manufacturing companies today are facing ever increasing competition. At the same time raw material costs continue to increase. These are factors that companies, for the most part, cannot control. Therefore companies must concentrate on what they can control: their processes. Companies must strive for continuous improvement in quality, efficiency and cost reduction. Many companies still rely only on inspection after production to detect quality issues. The SPC process is implemented to move a company from detection based to prevention based quality controls. By monitoring the performance of a process in real time the operator can detect trends or changes in the process before they result in non-conforming product and scrap.

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