SPC (Statistical Process Control)
Pareto Diagram
Although SPC is not as popular as it once was, statistical methods are the basis for sound process stability and capability. SPC was at one time a major initiative for quality in industry. It has had a proven track record in the USA and Great Britain during World War II and in Japan during the reconstruction. W Edwards Deming is given much of the credit for Japanese quality improvement post WWII by teaching and mentoring the Japanese engineering community in the proper use of SPC.
Process Stability is a primary desire of any industry, manufacturing, government or service. Consistent processes provide consistent quality good or bad. Stable processes can be improved further by applying the principles of capability. Control charts help the user understand what is likely to occur next and when an unstable condition takes place requiring study.
Control Chart methods are also taught in Six Sigma courses, as they are a great benefit for those attempting to learn about a process for the purpose of improving it.
Many methods exist but the most commons ones are listed below:
- X bar and Range
- Individual moving range
- Multi-variant control charts
- C, np and u charts for attributes
Each chart method is selected based on the desired information needed.
Capability
Capability is a term describing the processes ability to meet a specific specification. Indices such as Cp(k), Pp(k) describe how well a process is performing relative to the specification. For example: a Cp(k)= 1.33 means that the process is performing to a 4 sigma level to the closest specification or put another way, the side of the process distribution that is at most risk.
Cp(k) is used by many industries as a means of determining how well a process is performing. Key or Special characteristics are features that have the greatest benefit to customer satisfaction and are directly related to a customer expectation. Statistical Process Control (SPC) by its’ name means that the process and its’ parameters should be the primary items to monitor and control. Key or Special Characteristics which are dimensional, are one step away from the true critical control point which is the process. We believe a fervent effort should be undertaken to find the process parameters (energy) at the machine or station that affect the dimensional characteristics. Effort should also be taken to transfer control to the process and not the part.
Six Sigma Bell Curve
Other indices exist as well, each with their specific purpose and use. Cp(i), Pp, Cp(u) etc...
Quality Associates International has been and continues to be dedicated to the primary use of Statistical Process Control (SPC) for business improvement. These tried and true methods should never be swept away for the latest initiative. To support the use of SPC in industry, QAI provides training and mentoring in the proper understanding and use of SPC. SPC can bring value and the QAI team is experienced and dedicated to bringing the value of SPC to your industry.
Free SPC Downloads
QAI is now offering a FREE download of our SPC reference card. This card can be printed out for your reference. Download your FREE SPC reference card.
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