Five Costly Mistakes Applying SPC

Avoid these and amplify your QA efforts

I have daily conversations with manufacturer plant managers, quality managers, engineers, supervisors, and plant production workers about challenges when using statistical process control (SPC). Of the mistakes I witness in the application of SPC, I’d like to share the five most prevalent; they can be costly.

No. 1: Capability before stability

Capability is a critical metric, and capability statistics are often an important part of your supply chain conversation. Your customers want assurance that your processes are capable of meeting their requirements. These requirements are usually communicated as tolerances or specifications.

Customers frequently specify a process capability index (Cpk) or process performance index (Ppk) value that you must meet. Because they put such importance on this value, capability statistics may become your primary concern in quality improvement efforts. They may be important, but sole reliance on Cpk values is premature.

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Comments

Errors

The statement "Computing wrong limit values with a home-grown tool" is very misleading. As long as any "home grown" tool follows the guidelines laid down by Dr Wheeler, there is nothing to worry about. By contrast, many "professional" tools make blatant errors such as drawing meaningless and distracting distributions over histograms. Rather than warn about "home grown" tools, Steve should have warned about tools promoting "Six Sigma Analysis". This is simply farcical. https://www.linkedin.com/pulse/six-sigma-psychology-part-2-tony-burns Unlike his competitor, Steve's product does not fall into this bear trap.

Quality Digest is in the recycling business

This is much the same article as one from two years ago. Recycling is fine, but why not just reprint the one from 2015 and update the webinar info? Ditto for the recent articles from a columnist about "Gauging Gage ..." (I don't think the biography of that columnist is accurate now, as they are no longer at Minitab.) As for this article, the notion that "Time and time again I have seen examples where the numbers are just wrong, often resulting in audit failures" makes sense to me. A typo in a formula, treating a blank entry as a zero, copy/paste errors, SQL commands that aren't so structured, macros gone awry -- we could make a long list of possible hiccups.

Question

Steve,

This is some great insight to how careful you need to be when applying SPC in a plant. As a student near graduating and looking into working in a manufacturing environment soon, I now know to be mindful of these considerations. I think each of the 5 points is very important. If a process isn't predictable or stable, pushing it to maximum capacity is a huge risk. If you're using the wrong numbers to create the control limits, the control limits are useless; and control limits and specification limits need to both be considered to get the best results from both metrics, as both are important to successful production. SPC is also best utilized when practiced organization-wide, not just by a few individuals. That's when you'll start to see real impact. Also, with technology being so dominant in the manufacturing now, people are very reliant on these measurement machines and devices, so you made a great point about regularly checking if these machines are even accurately calculating is vital.

Clearly your 20+ years working with control charts and limits has impacted your knowledge of SPC. At what point in your career did you begin piecing these together and implementing them into your practices? Also, what are your thoughts on outsourcing consultants to come into the plant and implement these SPC practices? Have you found a lot of reluctancy in employees to listen to someone who doesn't work there?