Negative Stocks
Posted: Wed Apr 17, 2002 10:22 am
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>
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>
