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Clearing SPC Hurdles
Six obstacles threaten statistical process control success
by Scott A. Laman
Statistical process control (SPC) has provided significant cost savings for companies that are fortunate enough to implement it fully. Implementation challenges, however, can waylay the best of intentions.
When you consider the steps needed to begin using productive control charts, certain critical hurdles become apparent. These hurdles apply to all SPC programs, whether pencil and paper or automated, and whether simple or complex analytical techniques are employed.
Hurdle 1—Adequate training: After management support for the overall SPC program is obtained and the program’s purpose is publicized, training must be done. At least three levels of training are beneficial:
- Comprehension training for upper management, so the reasons for SPC can be defended, supported and tied to the bottom line.
- Competency training for engineers who are responsible for the processes and products that require the control charts.
- Awareness training for operators who are responsible for the day-to-day maintenance of the charts. Operators need to be able to collect the required data and react to what the resulting control chart data indicates.
The participants’ comprehension of the training should be verified, and rewards can be given to all who demonstrate an understanding of the techniques.
Hurdle 2—Process definition: The processes selected for SPC should be upwardly traceable to an important company metric, such as overall cost of quality, customer complaints or yield. Processes can be chosen based on whether they produce high percentage of sales or profit margin, or exhibit a gap between current and expected performance.
Hurdle 3—Variable selection: Critical process outputs need to be first identified, because SPC can be applied to these outputs to protect the customer. Typically, outputs are product characteristics, such as tensile strength, appearance or dimensions. Critical process inputs are then identified, often through an analytical tool or matrix that quantitatively allows prioritization by defining the importance and strength of the relationships between inputs and outputs.
Hurdle 4—Process stability verification: Process instability will create out-of-control points, leading to numerous corrective actions. If the process is not stable, an investigation should be performed to determine and correct the assignable cause. Careful data collection is of the utmost importance, as is expert analysis to ensure the absence of unusual trends and unexplainable data points.
Hurdle 5—Process capability determination: Once the process is demonstrated to be stable, the data can be compared to a specification. Capability indexes are calculated, and the results should meet company requirements. If the process is not capable, several options exist:
- Improve the process.
- Change the specification.
- Make a business decision to accept the low yield.
- Do not make the product.
Hurdle 6—Benefits demonstration: In cost-of-quality terms, SPC is a prevention cost. Implementing the program is an investment that will pay dividends later. The cost savings created by SPC should be calculated and publicized, stating reductions in:
- External failure cost of customer complaints.
- Internal failure costs of rework and process yield, including material and labor.
- Appraisal cost of final inspection.
Ultimately, a better understanding of a company’s processes through SPC should lead to higher profits, better job security and more opportunity for all employees. By identifying and clearing these hurdles, the chance of long-term success is much higher.
Scott A. Laman is a global quality systems senior manager for Teleflex Medical Inc. in Reading, PA. He earned a master’s degree in chemical engineering from Syracuse University. He is a senior member of ASQ and is a certified quality engineer, reliability engineer, quality manager, Six Sigma Black Belt and quality auditor.