Chaos Re-visited -- DickesonDecember 2001
You accept common cause variance as part of the system design. Either change the system design or abide by the range of common cause variance established by XmR control limits.
We must stop fretting and fuming about these variations. What we need to know is just when a variation exceeds the established control limits of the XmR charts and then pile on all our energies to eliminate those disruptions. The XmR charts tell us when to fret and when to relax, making us far more effective managers. It's Valium for production managers.
This all appears to fit nicely with Eliyahu Goldratt's Theory of Constraints. Chaos variations do constrain production throughput. Many times those constraints—design limitations—are the policies we've imposed for reasons of culture or habit. They're based on assumptions of how we think things work. Oftentimes those assumptions are wrong, or just outmoded. (Job cost accountancy is a prime example.) To "elevate" those system constraints (as Goldratt would put it), we must think deeply about them and be prepared to try some changes.
It may be that the variances (constraints) are physically imposed by equipment or workflow. If we must continue with them then we must find other means to increase throughput. As Wheeler expresses it, "We must listen to our process" and abide its constraints.
But, if we're working an 8/5 production plan, can we change to 16/5 or even to 24/7? But how shall we increase sales volume? Cut prices? Despite what my columnar colleague the "Mañana Man" says to the contrary, cutting prices may be the route to making a lot more profit. Ask the folks at WalMart. (Are they open 24 hours, seven days a week? Is Penney's? Is Sears?)
Either the glass is half empty and you just kick back, relax and sip to the last drop, or the glass is half full and you roll up your sleeves and create ways to fill it.
What's the new potion in this magic XmR bottle that enables us to approach print production more sensibly, more logically? "How, Don Wheeler, do I listen to my process?" Well, first of all it's not a new potion at all. A statistician named Walter Shewhart developed it back in the 1920s and his student W. Edwards Deming taught it to the Japanese to enable them to compete with us—without cost accounting, by the way.
We didn't have hand-held calculators and computers at the time, so Shewhart devised some simple shortcuts for calculating "control limits"—the "range" of expected variation. It's an application of "Central Limit Theorem" modified by Shewhart, preached by Deming and refined by Wheeler to use "Moving Ranges" of data.
These moving ranges, with their process limits, enable us to know whether we're still just feeling the butterfly wings of print chaos or whether some attributable cause has intervened. XmR charts teach us to expect and accept variation within limits. Still want to markup costs for a price to quote? XmR will tell you the best, worst and most likely result to expect. Want to use a D-B-R (Drum-Buffer-Rope) scheduling system? Then set your buffers and feed rope based on "What your process is telling you," not your dreams. Want to have on "on-time" delivery monitoring?
Well, go buy the book. Let Wheeler and some printer make a buck. Your process will talk to you. You may not like to hear what it says, but that's your glass to fill.
—Roger V. Dickeson
About the Author
Roger Dickeson is a printing productivity consultant based in Tucson, AZ. He can be reached by e-mail at firstname.lastname@example.org, by fax at (520) 903-2295, or on the Web at http://www.prem-associates.com.