Principle (Robustness): Pull is more robust than push.

Robustness refers to how well a system performs when its controls are set at less than optimal levels.  Since data is never accurate and systems are subject to constant change, controls are never set optimally in practice.  Therefore, robustness is absolutely key to making systems eaiser to manage.

As pointed out by the Observability Law, push systems are controlled by the release rate, while pull systems are controlled by WIP level.  But the Principle of Non-Linearity in Utilization implies that WIP level is very sensitive to release rate as we approach the capacity of the system. Since we want to set utilization high, in order to maximize revenue and amortize the cost of equipment, we will generally try to operate a push systems precisely in the region where it is most unstable.  Furthermore, since utilization is difficult to set because capacity is not easily observable, we are likely to set releases too low (and lose throughput) or even more likely to set releases too high (and swamp the system with WIP).  The net result will be poor financial performance.

In contrast, pull systems, which control the WIP level, are much more stable.  As we illustrated for closed systems, throughput tends to increase slowly in WIP level once the system is reasonably saturated.  This means that small errors in the setting of WIP level will have only a small effect on throughput, and hence on profit.
 
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