What is Statistical Process Control – QC Tools

Statistical process control (SPC) is a method of quality control which employs statistical methods to monitor and control a process. This helps to ensure that the process operates efficiently, producing more specification-conforming products with less waste (rework or scrap). SPC can be applied to any process where the “conforming product” (product meeting specifications) output can be measured. Key tools used in SPC include run charts, control charts, a focus on continuous improvement, and the design of experiments. An example of a process where SPC is applied is manufacturing lines.

Process Control:

Process control is a method of statistical quality control in which the quality of a product is controlled while the products are in the process of production. In process control, the control charts are used to detect the variations and control the quality of a product. Control Charts are the graphic devices for detecting variation in the production process and determining the permissible limits of variation. Control charts are based on the theory of probability and theory of sampling. Control charts consists three horizontal lines: central line, upper control limit, and lower control limit.

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.

statistical process control
statistical process control

Steps Involved In Using Statistical Process Control :

Plan – Identify the problem and the possible causes.

Do – Make changes designed to correct or improve the situation.

Study – Study the effect of these changes on the situation. This is where control charts are used – they show the effects of changes on a process over time. Evaluate the results and then replicate the change or abandon it and try something different.

Act If the result is successful, standardize the changes and then work on further improvements or the next prioritized problem. If the outcome is not yet successful, look for other ways to change the process or identify different causes for the problem.

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

Application of SPC

The application of SPC involves three main phases of activity:

  • Understanding the process and the specification limits.
  • Eliminating assignable (special) sources of variation, so that the process is stable.
  • Monitoring the ongoing production process, assisted by the use of control charts, to detect significant changes of mean or variation.
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