Negative Stocks

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"geoff coyle"
Senior Member
Posts: 94
Joined: Fri Mar 29, 2002 3:39 am

Negative Stocks

Post by "geoff coyle" »

Bill asks if treating negative stock explicitly as a backlog is a heuristic
or a firm rule and refers to his financial problems (donations can be sent
to me and Ill pass them on, less the usual commission).

Id suggest that it depends on whether or not behaviour changes. Someone
whose bank balance is only slightly negative probably makes no change to
behaviour as the next salary cheque is due in a few days. Someone else who
is seriously overdrawn at the bank really should change their policies on
spending. (It also depends on whether the bank charges more for overdrafts
than it pays on balances, but lets ignore that). Of course, one could
write:

POLICY1 if bank-balance > 0 (or a small negative number)

POLICY2 if bank-balance =< that number.

My personal style would be to model the overdraft (negative balance)
explicitly as that helps me to be clearer about what the model is doing in
the two cases and to check that against what it should be doing - debugging
it in other words.

>
> IOW, is non-negativity of stocks a heuristic that ranks below ensuring
> that stocks and flows represent real entities in the system that
> create the problem to be studied?
>

If you treat the negative stock explicitly as a backlog I dont think you
are violating that rule.

> > Incidentally, can anyone explain the origin of widgets?
>
> (I suspect the OED would be a better reference, but I dont have a
> copy.)

Why didnt I think of that! Collins dictionary has any small mechanism or
device, the name of which is unknown or temporarily forgotten. It derives
from gadget, which is of uncertain origin. Yet another challenging issue in
SD is laid to rest by British intellect, scholarship and modesty.

Geoff
From: "geoff coyle" <
geoff.coyle@btinternet.com>
Bill Braun
Senior Member
Posts: 73
Joined: Fri Mar 29, 2002 3:39 am

Negative Stocks

Post by Bill Braun »

I am starting a model of a large watershed. The question has arisen
regarding ponds that go dry (inches of water at pond beds low point = 0)
and then become parched below the physical pond bed (e.g., the typical
cracking of completely dried mud).

Rainfall in this case will not instantly add water to the pond, as the mud
will absorb water before accumulation above the bed of the pond occurs.

Is the pond a legitimate example of allowing a stock to go negative? Or,
would it be better modeling to have two stocks, one representing the
dryness of the pond bed (zero representing, for example, the water table,
below which the pond bed will not dry) which must reach a threshold of
saturation (i.e., x inches of rainfall) before the pond will accumulate water?

Bill Braun
Bill Braun <medprac@hlthsys.com>
Bill Harris
Senior Member
Posts: 75
Joined: Fri Mar 29, 2002 3:39 am

Negative Stocks

Post by Bill Harris »

geoff coyle wrote:
>
> Bill asks if treating negative stock explicitly as a backlog is a heuristic
> or a firm rule and refers to his financial problems (donations can be sent
> to me and Ill pass them on, less the usual commission).

The checks youre forwarding are _mighty_ small, Geoff--about dt in
magnitude. :-)

> Id suggest that it depends on whether or not behaviour changes. Someone
> whose bank balance is only slightly negative probably makes no change to
> behaviour as the next salary cheque is due in a few days. Someone else who

> My personal style would be to model the overdraft (negative balance)
> explicitly as that helps me to be clearer about what the model is doing in
> the two cases and to check that against what it should be doing - debugging
> it in other words.

As I recall (it was a small German "Sparkasse" in the early 1970s), the
account had a built-in loan. Thus, when I bought a car privately, the
way I was told to finance it at the time was to draw down my account and
let the automatic deposit of my paycheck build it back up--that was
supposedly a competitive interest rate _or_ the bank was taking
advantage of my financial naivete of the time. It wasnt an accidental
overdraft.

Thus, youre right; I pretty much behaved the same way as before, but
the bank deducted interest each month until the account was again
positive (just a couple of months, I think it was).

If I were to model that sort of entity in addressing a problem, Id
probably create one stock, for the account was indeed one entity. This
may be a corner case, I agree.

> > (I suspect the OED would be a better reference, but I dont have a
> > copy.)
>
> Why didnt I think of that! Collins dictionary has any small mechanism or
> device, the name of which is unknown or temporarily forgotten. It derives
> from gadget, which is of uncertain origin. Yet another challenging issue in
> SD is laid to rest by British intellect, scholarship and modesty.

Especially the modesty, eh? :-)

Thanks,

Bill
From: Bill Harris <
bill_harris@facilitatedsystems.com>
--
Bill Harris 3217 102nd Place SE
Facilitated Systems Everett, WA 98208 USA
http://facilitatedsystems.com/
"Jacob J Jacobson/JAKE/CC01/INEE
Junior Member
Posts: 2
Joined: Fri Mar 29, 2002 3:39 am

Negative Stocks

Post by "Jacob J Jacobson/JAKE/CC01/INEE »

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