The Misuse and Abuse of Process Behavior Charts

Nine common pitfalls to avoid

More than 30 years ago, I became aware of the power of statistical process control (SPC) and what we used to call control charts (now called process behavior charts). As technical director of the company where I was employed at the time, I promoted the idea of SPC, got upper management “support,” developed training materials, trained people, and waited for wonderful things to happen… and waited… and waited… and waited.

Since that time, I have seen many “improvement efforts,” “programs,” and “initiatives,” come in like a lion and go out like a lamb in various organizations. Until about 15 years ago, the lack of portable computer hardware and too few user-friendly software applications were the two primary barriers to widespread use of SPC. When I got started with SPC in the late 1970s, most of my work had to be done with a No. 2 pencil, a pad of paper, a slide rule, and the appropriate tables of bias factors. No wonder people were reluctant to institute the use of process behavior charts in the real world. It was a lot of work.

…

Want to continue?

By logging in you agree to receive communication from Quality Digest. Privacy Policy.

Create a FREE account

Forgot My Password

Comments

Another Pitfall - not understanding "important" shifts to detect

I/MR charts are frequently touted on this board as the best overall chart to start with (for continuous data). I prefer to ask the question, "how much of a process shift are we hoping to detect with our specific charting application? and then select an appropriate chart. Depending on the size of the shift we need to detect (may depend on current process capablity, or effect of variation on product performance) as well as the natural variation in the process - an Individuals chart may be completely inadequate to detect the types of process changes we desire to detect. In these cases, Xbar (if rational samples are possible) or CUSUM charts (among others) may be much more appropriate.

So....I suggest that a pitfall of SPC is not understanding or asking what the purpose of the chart even is? How much of a process change are we trying to detect (from a practical perspective). The chart design should align with these goals.