Use the Stock-Adjustment process whenever
you want to represent a flow of activity
which adjusts a Stock to a desired value. The Stock-Adjustment Process implicitly
represents a production flow: It describes the behavioral result of a production
process, but leaves unstated the resources and productivity required to actually
produce the adjustment flow. The three inputs to this process are the Stock (or its
alternate measure), the Desired Stock, and an adjustment fraction (or "time
constant"). The Flow is generated by multiplying the difference between the Stock
and the Desired Stock by the adjustment fraction (or dividing the difference by the
time constant). Whenever a discrepancy exists between the Stock and the desired
Stock, the resulting flow will gradually adjust the Stock toward its desired level. With
any Stock Adjustment Process, the flow can either increase or decrease the Stock,
depending on whether the Stock is greater or less than the goal. Both goal and
adjustment fraction are usually Converters, but can be Stocks.
Algebraic Form:
adjustment flow = (Desired Stock - Stock)*
adjustment fraction (units/time) = ((units)
- (units)) * (units/unit/time)
In some instances, you may want to think
of the adjustment fraction as a "time
constant." The time constant is simply the reciprocal of the adjustment fraction:
adjustment flow = (Desired Stock - Stock)
/ time constant (units/time) = ((units) -
(units)) / (time) where: time constant = I/adjustment time
Note that if the Desired Stock is 0, the
equation becomes:
adjustment flow = - (Stock * adjustment
fraction).
This is the equation for the draining
process ( the "-" sign indicates that the
Adjustment Flow would be an outflow -
which is what the draining flow is).
The behavior pattern of the Stock-Adjustment
Process, in isolation, is quite similar
to that of the draining process. The plot below shows how a Stock-Adjustment
Process, with a constant adjustment fraction and a constant Desired Stock,
behaves. The top curve is the pattern generated when the Stock adjusts from
above its desired level. The bottom curve is the pattern of stock-adjustment from
below. Both patterns reveal an asymptotic approach to the goal for the Stock. In
these examples, no overshoot of the goal occurs. None is possible with a single
Stock-Adjustment Process.
One common use of the Stock-Adjustment
structure is to represent an opinion,
perception, memory, or belief ~ something that is based on experience, and
changes slowly relative to the course of experience. For example, someone might
hold fond memories of a personal relationship long after the relationship has turned
sour. In a similar vein, some people act as if they are making lots of money, long
after they are laid off from their jobs.
As with the other generic processes, the
Stock-Adjustment Process presented
here assumes a constant goal and a constant adjustment fraction. The behavior of
this structure becomes more interesting when you allow goal and adjustment
fraction to vary. The examples below illustrate some uses of the stock-adjustment
structure.
