By Jon Miller | Post Date: September 16, 2007 3:40 PM | Comments: 12
I'm still scratching my head over an IndustryWeek article titled The Great Push vs. Pull Diversion by By Edward S. Pound and Mark L. Spearman of Factory Physics, consultants and writers of one of my favorite Lean books. They state:
Executives and managers in manufacturing have been subject to a great diversion ever since the advent of the Toyota Production System. In an effort to improve performance, many have wasted inordinate amounts of time and money in organizational struggles over push systems versus pull systems.
* If you are discussing the benefits of push vs. pull in your organization, stop.
* If you are implementing pull systems and focusing on achieving one-piece flow, know that you are in danger of leading your organization to decreased throughput performance, poor customer service performance, or both.
The authors ask the reader to "Understand that whether WIP is pulled or pushed is not the point and that too little WIP is as bad as too much WIP" and they make the points that we should not ignore the practical science underlying WIP levels in a push or pull system, and that this so-called "push versus pull diversion" is the result of confusion surrounding the definitions.
They proceed to criticize the definition of pull systems give by James Womack, and the fact that the supermarkets in which we shop do not appear to be a true pull system. The supermarket is a metaphor, not an operational model for a pull system. 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).
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. In fact, in some rare cases we have actually increased inventory levels in order to implement kanban systems.
In general terms, there is MRP-executed planned push production and kanban-driven downstream pull production. In a kanban-based pull system the quantity of kanban cards (and therefore WIP) is calculated based on usage, replenishment lead-time, container quantity, and something called the safety coefficient which takes into account the practical science of process variation mentioned in the article. The quantity of kanban cards in the system is adjusted based on the changes in monthly demand. The push system produces or moves product at the convenience of the producer process. The pull system only produces or moves product at the request of the downstream (customer) process.
Although a kanban system by definition must be a pull system, not all pull systems are kanban systems. You can keep your WIP levels exactly the same and convert form push to pull. You will stabilize your production output by stopping overproduction, as well as identify process problems. When you push, you are making production information and the shop floor condition obscure by de-linking material and information flow.
The key difference is in how information about consumption of the product is given to the producer process. In a pull system, material and information flow are linked. In a push system, they are not necessarily so.
The authors give two examples of companies facing risk through converting from push to pull (the billions saved going from push to pull must be folklore):
In blind pursuit of one-piece flow, many companies have driven throughput levels down to unprofitable levels. Since true one-piece flow is only possible with zero variability, and the laws of nature do not allow zero variability, true one-piece flow is an impractical ideal.
I would remind that all ideals, by definition, are impractical. If you achieve your "ideal" that just means you set your sights too low. Furthermore, since our clients have put one piece flow into practice, it may be a practical (that is to say attainable) ideal, at least to some.
The authors state:
Telling employees that they should "pull to demand" or "pull work only as needed" has a nice intuitive ring to it. Unfortunately, as we have discussed, intuition that is not fact-based can go badly wrong.
Actually we have found that the idea of pull is profoundly counterintuitive to the vast majority of people who we have introduced it to over the years. Pushing feels safer, more convenient, and the way things end up if you let entropy go (if you don't manage and improve systematically towards the ideal).
The authors are correct in that less WIP is not always better, but zero inventory and one piece flow still is the ideal. The Toyota Production System teaches us to improve towards the ideal, not to apply a theory based on practical science and be satisfied that push is as good as pull. If you are making changes without insisting on improving towards an ideal (takt, flow, pull, built in quality, etc.) then please don't call it Lean, or refer to it as TPS.
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.Comments are moderated to filter spam and inappropriate content. There may be a delay before your comment is published.