REPLY Age of material in a stock (SD6407)

[SDMAIL Jean-Jacques Laublé]

Posted by  Jean-Jacques Laublé <jean-jacques.lauble at wanadoo.fr>

Hi everybody.

There is an ambiguity in the question of Richard.
Does he mean, the average total time material was in the stock or average
delivery delay?
Or the current average age a material in a stock?
By the formulation of Little's law Richard probably means the first
formulation:
Average delivery delay = stock / outflow.
In equilibrium where inflow = outflow the average age of material in stock =
Average delivery delay / 2.
A good approximation of the average delivery delay is the maximum age of
material in stock.
If the distribution of the age is uniform in the stock, the maximum or
delivery delay will be the double of the average.
This way surveying the average age of material in stock by a co-flow gives a
good indication about how the delivery delay is moving if there is no sudden
change in the input or output process and the input approximately equals to
the output.
If the input is greater than the output, the non uniform distribution will
make the average age less than half of the maximum and the approximation of
the delivery delay by the average age in stock * 2 will underestimate the
reality, and the opposite if the input is less than the output.
But I did not find any way to calculate the precise average delivery delay
or total time in stock using co-flows.
The only way I found to calculate the average delivery delay is the model in
the Vensim forum.
Regards.
Jean-Jacques Laublé
Posted by  Jean-Jacques Laublé <jean-jacques.lauble at wanadoo.fr>
posting date  Tue, 17 Apr 2007 12:43:24 +0200