Definition of Push vs. PullIn a Push System, new input is determined by a plan or event. Output has to be adjusted accordingly.
In a Pull System, new input is determined by the system‘s capacity/capability. Input has to be adjusted to the output.
Pull Systems and WIP LimitsNow the connection between Pull and WIP limitation becomes evident. As Don puts it: "WIP limits are inherent to Pull Systems." If the input is to be determined by the system‘s capacity/capability, we 1) have to know this capacity/capability (therefore Lean‘s notion of studying the system and Understanding as one of Kanban‘s core values); and 2) we have to make sure that we never load the system beyond its capacity/capability. The easiest way of doing this is to only allow a new work item to enter the system, after another one has been finished. We have to "read" our system from right to left - just as we should "read" our Kanban board from right to left - hence the slogan Stop Starting, Start Finishing!
Pure Pull Systems?It‘s worth mentioning that pure Pull Systems probably do not exist. As Don Reinertsen points out in his brilliant presentation The Science of WIP Constraints, even the leanest system has a push-pull-boundary, meaning that the pull mechanism only starts after a certain process step. Before this step, work is pushed into the system. Even at Toyota, there is a minimum of planning and buffering involved - they don‘t melt new steel for every new car.
What‘s probably more important to knowledge work is the fact that want to achieve as much pull as possible in our system, but we also want our system to be able to absorb some push. Sounds strange, but it enables us to cope with major unforeseen events. In Kanban lingo, most expedite tickets will be pushed into the system. Ideally, our system is under-utilized, so that it provides spare capacity to deal with this extra work. But even if it does not, we might be willing to accept the push, because the cost of waiting for a free slot would be much higher than the cost of temporally overburdening the system. But that should be discussed further in a separate blog post...
Like this post? Then you should check out my previous post Keep the Ball Rollin‘