The Push vs. Pull Diversion Diversion

I have read Factory Physics, and i know it is obvious (by the practical science discussed in this book) that Pull system is superior to push. it was clear enough. but they have written about WIP in more details than other books i have ever seen. as you have absolutely know, in F.P., WIP is being capped, then it will be reduced by decreasing of variability. most of technics it is used to decrease variability, are in the lean tools packages plus continuous improvement.
as i see, both of them – traditional lean books and factory physics – accept pull system is better than push system, however it is implemented.
have i been misunderstood?

If Dr. Spearman and I have been unclear in our explanation of the use of push vs. pull terminology in our Industry Week article (http://www.industryweek.com/ReadArticle.aspx?ArticleID=14900), hopefully we can clear up any misunderstanding. Also, Mr. Miller has made a number of misstatements about the article content and I would like to address those here.
First, some background. I have over 18 years manufacturing experience and, until I started working with Factory Physics Inc., had spent all my professional life in manufacturing operations and management. This has included:
– two and a half years in Japan working in production at a Japanese company—Kaizen was a daily occurrence
– Lean transformation work with Shingijitsu at AlliedSignal (now Honeywell)
– Supervision of Six Sigma efforts at a line management and executive management levels.
Dr. Spearman has over 29 years experience in research and consulting with manufacturing companies and as an award-winning professor at Northwestern University (where I was a student of his) and at the Georgia Institute of Technology.
We understand the use of the classic concepts of push and pull and have used them in practice. Our work with Factory Physics concepts has shown us that there are improvements to be made in the terminology, understanding and practices of operations productivity improvement which led us to write the Industry Week article.
The following is incorrect, “Edward S. Pound and Mark L. Spearman of Factory Physics, consultants and writers of one of my favorite lean books.” Mr. Miller doesn’t name the lean book but the assumption here is that Mr. Miller is referring to the book Factory Physics.
– The book “Factory Physics” was written by Mark Spearman and Wally Hopp. I was not one of the authors.
Mr. Miller states: …”The author’s don’t offer an authoritative definition of push vs pull (perhaps because this is a discussion we should stop having, according to them).”
– In fact, in the article is the following: “A clear definition states, ‘A push system schedules the release of work based on demand while a pull system authorizes the release of work based on system status.’” This definition is out of one of Mr. Miller’s favorite books, “Factory Physics” 2nd Edition, page 162.
– The book “Factory Physics” is a fairly authoritative source. It is widely used in industry and at over 100 academic institutions to teach the practical science behind manufacturing management. It was also selected as Book of the Year by the Institute of Industrial Engineers shortly after it was published. A third edition should be coming out at the end of this year or in early 2008.
Mr. Miller states, “Let’s clarify: the difference between push and pull is not between the quantity of WIP in the system. That is simply one of the end results of converting from push to pull.”
– This is exactly the kind of statement that causes emotional diversion discussions that the article was addressing. The article states,
o Do not be diverted by underproductive discussions of push versus pull. The practical science that governs your operations performance is not as simple as push versus pull.
o WIP control is a strong determinant of performance and, in the real world, less WIP is not always better.
– The article never states that the difference between push and pull is the quantity of WIP in the system. The article states that WIP control is a fundamental control for determining operations performance so whether you call it push or pull is irrelevant. Since the discussion of push versus pull typically leads to these type of discussions, we recommend companies talk about WIP control, understand the science behind WIP control and avoid this type of debate. Our experience with leading companies around the world has shown that this leads much more quickly to achieving the results that companies want. After all, companies don’t want to be the leanest or have the lowest variability or to be like Toyota, companies want to make profit over the long term. Toyota is just one company that has achieved long term profitability.
Mr. Miller’s last statement exemplifies the type of approach we are trying to avoid, “Please continue to have this discussion until it is clearly understood that pull is superior to push, and then return to your study of the practical science of Edward Pound and Mark Spearman.”
– Simply saying, “Believe what I say,” is not a learning and improvement approach. We are saying that the use of the terms push and pull is not the point. WIP control and the mechanics of WIP control are determinants of operations performance. So understanding the practical science that describes WIP control is a competitive advantage in improving operations performance. If an approach is to be objective and productive, conjecture and refutation are encouraged to ensure the usefulness and credibility of the approach. To quote Daniel Boorstein, “The greatest obstacle to discovery is not ignorance—it is the illusion of knowledge.”
– Finally, we are major supporters of good Lean Manufacturing practices. Indeed, many companies have been very successful with Lean Manufacturing techniques. Both Dr. Spearman and I have worked successfully with many companies’ Lean Manufacturing personnel and we are strong supporters of good productivity practices. It is our experience that the use of the terms push and pull are typically loosely defined and tend to divert operations strategy discussions, manufacturing logistics design efforts and operations productivity improvement efforts towards the type of discussions illustrated here whereas a clearly defined practical scientific approach moves companies more quickly towards the results they desire.
For an even more thorough discussion of the history of push versus pull, go to the following link: http://www.factoryphysics.com/member.cfm?mode=default&view=whitepapers and download the white paper titled, “To Pull or Not to Pull, What is the Question?” A two minute registration process is required. This paper was commissioned, refereed and appeared in the journal of Manufacturing and Service Operations Management in 2004.
Sincerely,
Ed Pound

Dave is correct — Factory Physics (the book) does discuss push vs pull in some detail, but the key is limiting WIP. Most push systems (run by ERP or otherwise) assume infinite capacity and will “stuff the goose” by continuing to cram more into the system, which increases WIP beyond the optimal level and increases CT.
The “CONWIP” system (as described in the book) is a hybrid push/pull system, were you only start something new into the process when something has finished, but you might “push” within the process (say an assembly line). Having “flow” in a “push” process that has its WIP limited by the CONWIP rules (a different type of pull, since the start into the process is authorized by a card/signal) can be very efficient. The Factory Physics rules help you calculate the optimal WIP for the given system. But, they don’t assume the system’s variation is a given (as push manufacturers would), they would recommend reducing variation by using Lean and Kaizen methods.
Let’s not bow down at the altar of “pull.” What’s Womack’s phrase, “flow where you can, pull where you must”?? Pull isn’t a cure all and pull isn’t the right approach for all systems (try it in semiconductor fabrication… not practical).
I think the article, and Pound/Spearman, are correct that adopting “single piece flow” pull systems without first addressing the underlying variation can be suicidal to a business. I’ve seen it first hand.
(Full disclosure, I was a student of Spearman’s as an undergrad at Northwestern, but my defending them isn’t out of personal loyalty, but out of a belief that their ideas are correct).

A toyota assembly line is not strictly “pull.” A continuously moving line is a “push,” one could argue, no?

Jon,
A few more points:
> TPS is not the target. The target is to be profitable over the long term. Toyota is just one company that has been profitable over the long term.
> All of the discussion after my previous posting reinforces the point of the Industry Week article. As an exercise, try describing how an assembly line works without using the terms push or pull. For instance, Toyota and most all automotive assembly lines keep WIP and cycle times low because the stations on the assembly line limit the amount of WIP that can be in the line at any time. Tact time is tied to demand and setting the assembly line speed paces all operations to tact time and thereby provides throughput to meet planned demand (note that planned demand is almost never equal to actual demand). If the assembly line hits the tact time day in and day out, cars will be produced to planned demand with minimum cycle time and minimum amounts of WIP. Of course, we know from Little’s Law and other basic Factory Physics principles that WIP and cycle time are essentially equivalent so limiting WIP limits cycle time. Note that this description of the basic logistical performance of the Toyota assembly line was completed without using the p words.
This is pretty straight forward for automotive assembly lines but the simplicity of the logisitics does not appear as well in most other industries where there is no assembly line to:
> provide a pacer for workflow
> act as a cap on WIP
Then, the problem becomes determining what level of variability is acceptable and, given that level of variability, determining the minimum amount of WIP required to get maximum throughput (the Optimal WIP zone).
Thanks for the opportunity to address these issues.
Regards,
Ed Pound

Hi Mark,
Those buffer zones are more like hand-off zones. If I recall correctly, there are no extra vehicles in these zones to act as “buffer”. The hand off zones are there to allow the line to keep moving even if an andon cord is pulled, so long as the problem is solved before exiting that hand off zone.
The Toyota assembly line is a fixed position stop system rather than a continuous push conveyor or a pull-started conveyor. Even if there was one piece of buffer, if the line remains stopped unless that piece moves, it is a pull system. I’ll ask our Toyota guys for clarification.
Yes, the kanban system using a supermarket is a pull.

“The authors are correct in that less WIP is not always better, but zero inventory and one piece flow still is the ideal.”

This is entirely dependent on the business. If the business requires rapid capital recovery and has negligible holding costs it operates completely differently from a company with massive holding costs and very slow capital recovery. I’ll let yall do the math yourselves on how each of those businesses should run.