Subject: Connections between models and reality

Posted by Mohamad Morovati on 3/6/2004
In Reply To:Connections between models and reality Posted by Niall Palfreyman on 3/5/2004

Message:

Stocks are accumulated during a period of time, so they can be seen. but Flows are the rate that stocks change, so they can only be seen over a period of time, thus flows usually are invisible unless you view them in a period of time.

as a result, there may be a good criterion to distinguish between Stocks & Flows: if you take a photograph, you can only see the stocks of a system. those variable that need the presence of time to be understood, are flows.

think of population which is a stock. you can see it, count it, take a photograph... but what about birth rate? we cannot exactly see the thing. we just count the newly born babies during a period of time and then divide it by the period length to extract the rate. this means that: we can not see the birth rate, we just have it in mind.
to say it in brief, stocks are those things that can be measured dirctly , but flows are measured indirectly(through stocks).

this is the main reason that all stocks have got names but many of flows do not have a name. they haven't been noticed because of their being invisible.

consider the Price of something[which is a stock] and "the rate of change of the price"[which is a flow]. the price has got a name but the rate hasn't.

I can summerize the whole thing in few sentences:
if you take a photograph, you see the stocks.
if you let the time pass, you can distinguish the flows.
stocks are directly measurable but flows aren't.(they should be measured through stocks odf the system.
most of flows do not have any names because they can't be distinguished at a glance.

Follow Ups:

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