Golden nugget 4: Inventory target, Asymmetric time constants

Dr. Eliyahu M. Goldratt

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When experiencing a decline or increase of consumption for an SKU, the inventory target should be changed accordingly. But many times a change in consumption is merely a statistical fluctuation and not a trend, and therefore should not trigger an inventory target change. Trouble is, when experiencing a lower or higher consumption, we cannot foretell if it signifies a long term change that requires action (changing inventory target), or whether it is just a fluctuation. We want to react quickly to real changes, but not to noise: If we react too fast, many noises will be treated as real changes, and we will cause undesirable oscillations in the system – frequent changes of the inventory target, back and forth. On the other hand, if the reaction will be delayed, the system will meanwhile suffer from either too high or too low inventory levels. Obviously, we need to decide on a suitable time constant for responding to change – determining for how long should the buffer stay redgreen before we react by changing the inventory target.