Statistical Process Control (SPC) is a continuous improvement strategy used for quality control in manufacturing. As the name suggests, this methodology uses statistical methods to monitor and control processes to ensure all production processes are operating as efficiently as possible. The SPC methodology was developed in the early 1920s by engineer and statistician Walter Shewhart. The strategy was used over the next few decades, throughout World War II, and eventually was introduced to the Japanese manufacturing industry. SPC is now a popular concept in Six Sigma and other quality management organizations.

Applying SPC to examine processes typically involve three phases of activity:

  1. Understanding the process and the specification limits.
  2. Stabilizing the process to eliminate sources of variation.
  3. Monitoring the ongoing production process to detect significant shifts in variation.

Control charts, also known as Shewhart charts, are a critical SPC tool in that third phase. The use of control charts or related graphs is not a one-time thing, but rather an ongoing process to plot variations over time. Data is collected, organized, and analyzed to help track any major changes in variation, serving as an indicator of issues. The control chart helps to distinguish two types of process variation:

  • Common cause variation: Variations that are to be expected within a process and will typically always be processes. These process variations are considered ‘natural’ or ‘common’. Processes that fall within common cause variation “in control” in the realm of statistics.
  • Special cause variation: On the other hand, these process variations have larger amounts of variation, usually stemming from external sources, and are considered “not in control.” Special cause variation is especially wasteful and reducing the amount of variation in the manufacturing process is key to operating efficiently while continuously improving the facility.

The middle of the graph is the area where processes operating within common cause variation and the top and bottom of the graph represent special cause variation, processes performing outside of “typical” process performance.

The control chart is a specific tool that managers can use to monitor processes and continuously improve their processes to reduce variation. SPC will help ensure quality products and can be used to reduce and eliminate wastes in the manufacturing line as well as reduce the likelihood of the finished product needing to be reworked or scrapped.

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