REPLY Age of material in a stock (SD6404)

[SDMAIL Will Glass-Husain]

Posted by  "Will Glass-Husain" <wglass at forio.com>

Dear Richard,

Good responses by both Jean-Jacques and Jack.

Actually, I'd say that the technique to estimate the age of materials in a
stock depends on the type of stock.

For a homogeneous pool of materials, e.g. a classic "bathtub", then Jack's
point is absolutely correct.  Use a coflow structure to calculate the
average time the material comes into each stock, then subtract from current
time to calculate the average length the material is in the stock.

For a stock that represents a more sequential process, e.g. a shipping
process, or a project management process, then the average time approach
often doesn't make sense.  The average time calculation will be incorrect if
tasks in a project are executed in a FIFO (first-in-first-out) order inside
the stock.  This is more visible with some behavior patterns then others.
(for example, an inflow that shuts off completely will show a declining
instead of constant time to process while material is still in the stock).

The iThink/STELLA software has a sophisticated cycletime capability, which
trackes "batches" of items flowing into the stock at each DT.  The modeler
clicks a stopwatch at an upstream flow which invisibly timestamps each
batch.  The actual cycletime (amount of time from stopwatch to current time)
can then be reported at any point downstream.

You can track the same information using a parallel stock/flow structure
that is arrayed over time.  I've created this using Forio Brodcast's
simulation language but the concept would work with any modeling tool that
supports arrays.  The behavior of this model exactly replicates the results
from the iThink cycletime function.

The link below is a simple web page showing one specific simulation run.
The model can be downloaded and applied to any stock/flow structure.

http://forio.com/simulation/cycletime/

Best regards,

WILL

-- 
Forio Business Simulations
Posted by  "Will Glass-Husain" <wglass at forio.com>
posting date  Mon, 16 Apr 2007 11:14:49 -0700