average time in stock model

[LAUJJL]

Hi

Here is joined the model under discussion on the SD mailing list, answering the late question from Richard Dudley.
The model joined is now right.
Previous was not.
Regards.
JJ

[Edited on 16-4-2007 by LAUJJL]

[LAUJJL]

the question from Richard was:
Little's law can tell us about the length of time material is in a stock
when the stock is in equilibrium.

Is there any way to _estimate_ the the length of time material is in a stock
when that stock is not in equilibrium? Or is this a complete unknown?

[ For those who don't recognize it - as I didn't - Little's law is simply

stock = average delivery delay * inflow

which I tend to think of as Jay's formulation that

average delivery delay = stock / outflow

where outflow = inflow in steady state. Jay's formulation is always
exactly true if the underlying delay distribution is exponential
(outflow = level/delay) - if not then only in equilibrium just like
Little's Law.]

Richard

[LAUJJL]

I downloaded a wrong version of the model.
The right one is joined.
JJ

[LAUJJL]

joined the generalized version that calculates the average time in stock depending on the time when was inputed the material in the stock.
Once the precision wanted is reached the value stays stable.
the subscripts represent the time when the input was introduced.
The size of the subscripts correspond to the length fo time you want to survey the average time in stock.
Regards.
JJ

[bob@vensim.com]

If the material follows a pure FIFO (First In First Out) process then using either ithink's cycletime mentioned in the mailing list or Vensim's QUEUE AGE OLDEST function will give the exact throughput time (QUEUE AGE AVERAGE for the average age of material in place).

These functions do not use subscripts - but really all they do is create an internal array material with time stamps on when they came into the stock.

[LAUJJL]

Joined a third model, that includes a co-flow that calculates the As, average time in stock, that is equal to the To, total time in stock, when the input equal the output and there is a perfect mix of the elements in the stock, relative to the probability to be chosen for the output, contrary to the Fifo
organisation.
See the discussion in the SD mailing list.
Regards.
JJ