At the time Mr Taiichi Ohno, architect of the Toyota Production System, stated we should no longer make large amounts of stock and then try to sell it (push), but need to be aware of the market (pull), this meant a turnaround in our thinking. Nowadays, “push” and “pull” are two of the most used terms in SCM. So far, so good. Nevertheless, what troubles me is that everyone defines push and pull differently. The apparently logical words of Mr Ohno are obviously not unambiguous.
A couple of examples of various schools:
Pull is kanban and push is MRP?
Pull was quickly associated with its first manifestation, kanban. At the same time push became synonymous with MRP. Although kanban was just the means to an end, many believed that kanban / pull production and TPS were synonym.
Pull is make-to-order and push is make-to-stock?
The original words of Mr Ohno have later been translated more than once:
- Pull in simplest terms means that no one upstream should produce a good or service until the customer downstream asks for it, … (Womack & Jones, 2007)
- Pull production is the opposite of push. It means products are made only when the customer has requested or pulled it, and not before (The Boeing Company, 2002)
Logical that push and pull meanwhile are being used extensively as synonyms for make-to-stock and make-to-order. The order decoupling point is even known as the push/pull point. According to these definitions, a ‘make-to-order MRP-system’ would make an ideal pull system. Question is whether Mr Ohno meant it this way.
Pull is observe & respond and push is plan & execute
According to another school (Toshiki Naruse (2003) and De Toni, Caputo and Vinelli (1988)) the distinction between push and pull is based on what initiates the purchase of parts or the start of production. Push if it is initiated by planning (plan & execute); pull if it takes place in reaction to the actual situation (observe & respond).
Pull limits the amount of work in process; push not?
Hopp and Spearman (2003) define a pull production system as a system explicitly limiting the amount of wok in process (WIP); unlike a push production system. The good news is that according to their definition, pull can be implemented in different ways. Kanban is a technique to limit WIP, but there are more possibilities, CONWIP, POLCA, but also MRP with a WIP-limit.
Pull defined as ‘following customer demand’ is emotionally superior to push. I suspect the advocates of Lean to have smartly used this emotion by consequently posting it as ‘intelligent pull’ and ‘senseless push’. Smart, but questionable. That is, MRP can easily be described as pull, ‘following customer demand’. After all, the starting point of MRP is that you only produce if there is a calculated requirement based on independent demand. Moreover, you could challenge whether it is sensible to follow customer demand. With few exceptions customer demand is variable; sometimes more, sometimes less. Following this variability one-to-one can have a deteriorating effect on capacity, material availability and throughput time. Especially if your customer asks you to deliver fast. Calculate your profit.
Obviously, you don’t have to tell Mr Ohno that it’s better to level demand variability with inventory and/or lead time than to follow demand one-to-one. During busy times you can draw from capacity stock and during calm times you can replenish your capacity stock again. Sounds a lot like push, doesn’t it?
Finally, it is without any doubt that observe & respond is preferable to plan & schedule and that systems controlling WIP are more reliable and controllable than systems controlling output. But why would you need to call a system based on observe & respond or one that limits WIP a pull system? Puzzles me.
Stop the confusion
Are you still with me? According to me the terms push and pull are just creating confusion. Maybe deliberately caused by Mr Ohno who might has put us on the wrong track by sketching an apparently logical but half truth about the secret of his success.
I therefore propose not to use the terms push and pull in any other meaning but the ordinary ‘shoving’ and ‘towing’. Please, just say what you mean: kanban or MRP, inventory- or order controlled, observe & respond or plan & schedule, even forward or backward scheduling.
Finally, I would like to advise you to learn about the importance of WIP control. Because that, as Mr Ohno already knew, holds the true secret of controlling your supply chain. Whether you can do that better with kanban, CONWIP, POLCA or another variant depends on your situation. And there is no word push or pull in that.