A few weeks ago I was in a meeting with a quality engineer from a Fortune 500 company who had run a process capability study on some supplier data. In this particular case we were dealing with a one-sided specification limit (LSL), and the engineer used an Excel spreadsheet to calculate a Cpk of -3.4, which we all know is an outrageous number.
I asked to see the data and calculation, and the first thing I noticed was that the process average was above the lower specification limit, which told me that there was no way the Cpk could be negative, since in this case the Cpk calculation is Cpk = (Mean – LSL) / (3 sigma). If the mean was greater than the LSL, how could the Cpk be negative?
The answer was that the engineer was using a PPAP workbook to calculate Cpk, but did not understand the Cpk formula, and the PPAP workbook was not set up to deal with one-sided specification limits. Fortunately in this case the supplier understood that the quality engineer had made a big mistake, and quickly corrected him.
The result of the quality engineer presenting such an erroneous Cpk calculation resulted in a complete loss of credibility with the supplier, along with the other engineers from his own company who were in the meeting.
I’ve since had more time to work with this quality engineer, and it’s clear to me that he has a dangerous habit that his holding back his career and possibly putting his job in jeopardy – he does not absorb technical details. It’s not that he isn’t capable. He has simply benn in the habit of mentally “checking out” during meetings, and apparently in much of his past training.
There are two lessons here, I believe, for all of us –
(1) Know the critical details of your field thoroughly. For example, any quality engineer should be able to recite the Cpk formula without having to look it up.
(2) Never stop learning – as long as you are stuck in a meeting or training session, you might as well “turn on your brain” and absorb/understand all the details! It’s much more rewarding that daydreaming.