One of the simplest and most powerful visual controls is the horizontal line. Placed strategically above a stack of inventory, across a graph plotting daily quality performance or even to indicate a safe height clearance for vehicles, the horizontal line can give us information of great value for relatively little effort.
But just as knowing where to place the X on the treasure map is much harder than the actual act of marking the X itself, identifying the standard in terms of normal or target condition can be almost as hard as regularly achieving it.The best way is just to get started by marking the line at a safe level and gradually lowering it.
The metaphor of the water level is commonly used in lean thinking, particularly related to reducing inventory or lead times. The water represents the excess stock or time that dampens the effect and hides the problems that cause them. Many companies feel this condition of having out-of-control-but-buried problems is “normal” and maintain high levels of excess resource. Lean transformations achieve breakthrough results in part by setting these water levels, exposing the problems that are exposed, and resetting these water levels lower.
People commonly question the practices of Just In Time, stock less production or one piece flow as being unachievable, unreasonable in some business environments. The correct way to understand these is to see their dual purpose. On the one hand the synchronization through JIT sets the stocks at the right levels at the right places, often less than before JIT. On the other hand JIT sets a water level low enough to make the rocks beneath the water visible at a safe distance. The problems are not “out of sight and out of mind” but at the same time they are not an immediate risk of shutting down the line. Many a kaizen event has can left a one piece flow layout in its wake without giving people neither the tools nor the understanding that the water level must be dynamic, mirroring the supply, demand and process reliability parameters.
Just like the water level label in this hotel bathtub is only a standard based on average conditions, we need to be thoughtful of the water displacement based on the volume of the customer in the tub and how much they will splash around.