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Principle (Arrival Variability): For a fixed throughput level, inventory and flow time at a station are increasing function of the arrival variability to the station.

Variability at a station comes from two sources, the process variability, which is created at the station itself, and the arrival variability, which is variability passed on from an upstream station.  We can think of arrival variability as measuring the 'burstiness' of the arrival process. The following figure illustrates the difference between low variability (smooth) arrivals and high variability (bursty) arrivals.
 

The line represents time and the dots represent the point in time where an arrival to the station occurs. The arrival to the first station in the line could be from the predefined release rate or external arrivals (as in the case of arrival of orders). For intermediate stations the arrival process is the output process of an upstream  station.  By the Variability Propagation Law, process variability at one station turns into arrival variability at another station.

The appropriate measure of arrival variability is the coefficient of variation (CV) of the interarrival times.  The following figure illustrates the on flow time and WIP of increasing the CV of interarrival times to a station, given a fixed throughput level.  .
 

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