AOU What Is Meant by Stock and Flow and How Does the System Dynamics Use a Particular Diagramming Discussion Don’t forget the drawing Take into account th

AOU What Is Meant by Stock and Flow and How Does the System Dynamics Use a Particular Diagramming Discussion Don’t forget the drawing

Take into account the number of words

You will need Chapter 6 & 13 Figures and Tables excerpted from
Business Dynamics:
Systems Thinking and Modeling for a Complex World
Chapter 6
John D. Sterman
Massachusetts Institute of Technology
Sloan School of Management
Figures and Tables excerpted from
BUSINESS DYNAMICS: SYSTEMS THINKING AND MODELING FOR A COMPLEX WORLD
John D. Sterman
Published by Irwin/McGraw-Hill, an imprint of the McGraw-Hill Companies, Inc., 1221 Avenue of the Americas, New York, NY 10020.
Copyright ? 2001 by the McGraw-Hill Companies, Inc. All rights reserved.
The contents, or parts thereof, may be reproduced in print form solely for classroom use with
BUSINESS DYNAMICS: SYSTEMS THINKING AND MODELING FOR A COMPLEX WORLD
provided such reproductions bear copyright notice, but may not be reproduced in any other form or for any other purpose without the prior written consent the
McGraw-Hill Companies, Inc., including, but not limited to, in any network or other electronic storage or transmission, or broadcast for distance learning.
Esra Alyaheby
6.1 STOCKS, FLOWS, AND ACCUMULATION??????? ????????? ?????????
Causal loop diagrams are wonderfully useful in many situations. They
are:? ?? ?????.????????? ??????? ???? ????? ????? ?? ?????? ?? ????????
:
Well suited to represent interdependencies and feedback processes. ?????? ??? ??? ?????? ???????
o
.????????? ???????? ???????? ????????
Used effectively at the start of a modeling project to capture mental models ??????? ??????? ???
o
?????? ????? ??????? ??????? ??????? ????????
– both those of a client group and your own..???? ?? ?????? ??????? ?????? ???
o Used to communicate the results of a completed modeling effort. ??????? ??????? ????? ????
.???????? ?????????
However, causal loop diagrams suffer from a number of limitations and can easily
be abused..? ????? ???????? ???????? ??????? ?? ??? ?? ?????? ????? ????? ????????? ???????? ???? ????
o One of the most important limitations of causal diagrams is their inability to
capture the stock and flow structure of systems. ???? ??? ?????? ??? ???????? ????????
.??? ??? ?????? ??? ?????? ???? ??????? ??????? ????????
Stocks
and flows, along with feedback, are the two central concepts of
o
dynamic systems theory. ? ??? ????????? ??????????? ? ??? ???? ??????? ????????? ??????? ??????????
.??????? ????? ????????????
Esra Alyaheby
6.1 STOCKS, FLOWS, AND ACCUMULATION
•
Stocks
o They are accumulations (Buildups)..)??? ??????? (??????
o They characterize the state of the system and generate the information
upon which decisions and actions are based.
o .????? ???? ???? ?????? ?????? ????????? ???? ????? ????? ???????? ???????????
Stocks give systems inertia and provide them with memory. ??????? ?????
o .?????? ?????? ??????? ?????????
Stocks create delays by accumulating the difference between the inflow to a
process and its outflow. ????? ?????? ??????? ?? ???? ????? ????? ??? ?????? ??? ????????
o .???????? ????????
By decoupling rates of flow, stocks are the source of disequilibrium
dynamics in systems.
.? ??? ????????? ?? ???? ???????? ???????? ?? ??????? ??? ???? ??? ?????? ???????
Esra Alyaheby
6.1 STOCKS, FLOWS, AND ACCUMULATION
Stocks and flows are familiar to all of
us.??????? ????????? ?????? ??? ??????
The
. inventory of a manufacturing firm is the stock of product in its warehouses. ???? ???? ????????
??? ????? ?????? ?? ???????????
The number of people employed by a business is a stock. ???? ??????? ????? ?????? ??? ???? ?? ???
.?????
The balance in your checking account is a stock..??????? ?? ??? ?????? ????? ?? ?? ???????
Stocks are altered by inflows and outflows..???? ????? ?????? ?? ???? ???????? ??????? ?????????
o
A firm’s inventory is increased by the flow of production and decreased by the
flow of shipments..?????? ????? ?????? ?? ???? ???? ??????? ?????? ????? ????????
The workforce increases via the hiring rate and decreases via the rate of quits, layoffs,
and
o retirements. ? ???????? ?????? ????? ??????? ?? ???? ???? ??????? ?????? ?? ???? ???? ??????? ?? ??????
.? ?????? ????????? ????????
Your bank balance increases with deposits and decreases as you spend. ?????? ??????
.o???????? ?? ??????? ????? ????? ???????
Yet despite everyday experience of stocks and flows, all too often people fail to
distinguish clearly between them. ? ? ??? ????? ?? ?????? ??????? ????????? ??????????? ???? ????
.??????? ?? ???? ????? ?? ??????? ????? ??????
Failure to understand the difference between stocks and flows often leads to
underestimation of time delays, a short-term focus, and policy resistance ????????
? ? ???????? ??? ????? ???????? ??? ???? ????? ?? ??? ????? ??? ?????? ????????? ??? ??????? ?? ??????? ???????
.???????? ????????
Esra Alyaheby
6.1 STOCKS, FLOWS, AND ACCUMULATION
6.1.1 Diagramming Notation for Stocks and Flows ???????? ???????? ?????? ??????????
System dynamics uses a particular diagramming notation for stocks and flows. ??????? ???????? ???????
.?????? ?????? ??? ??????? ??????????
Stocks are represented by rectangles (suggesting a container holding the contents of the
stock)..)???? ????? ?????? ?????? ???????? (???? ??? ????? ????? ??? ??????? ????????
Inflows are represented by a pipe (arrow) pointing into (adding to) the stock.
Outflows are represented by pipes pointing out of (subtracting from) the stock. ?????
? ??? ????? ???????? ???????? ????????.?????? ???????? ?????? ????? (???) ???? ??? (????? ???) ????????
.????? ??? (??? ??) ????????
Valves control the flows ??????? ?????? ?? ?????????
Clouds represent the sources and sinks for the flows. ????? ?????? ??????? ?????????
.??????? ??????????
A source represents the stock from which a flow originating outside the boundary of the
model arises; sinks represent the stocks into which flows leaving the model boundary drain.
????? ?????? ??????? ???? ???? ??? ???? ???? ???? ??????? ? ???? ??????? ????????? ???? ??????
.?????? ????? ??????? ??????? ?????????
Sources and sinks are assumed to have infinite capacity and can never constrain the flows they
support. ??? ??????? ?? ????? ??????? ???????? ????? ??? ?????? ??? ?????? ?????? ????? ?????????
.????? ???????
Esra Alyaheby
6.1 STOCKS, FLOWS, AND ACCUMULATION
6.1.1 Diagramming Notation for Stocks and Flows
The structure of all stock and flow structures is composed of these
elements.?????? ???? ???? ????? ??????? ??????? ?? ??? ????????
.
Esra Alyaheby
6.1 STOCKS, FLOWS, AND ACCUMULATION
6.1.1 Diagramming Notation for Stocks and Flows
As the example in the figure shows, a firm’s inventory is a stock that accumulates
the inflow of production and is reduced by the outflow of shipments. These are
the only flows considered in the model: unless explicitly shown, other possible flows
into or out of the stock, such as inventory shrinkage or spoilage, are assumed to be
zero. ? ??? ???.? ??? ????? ?????? ?? ????? ?????? ???? ??????? ????? ?? ???? ????????? ???? ?? ???? ?? ??????
? ?????? ?? ???? ?????????? ? ?? ?? ??? ????? ??? ??????:????????? ??????? ???? ?? ????? ?? ???????? ?? ????????
.? ? ?????? ? ??? ?????? ??????? ?? ??????? ????????? ?????? ???? ?? ???? ????????
The clouds indicate that the stock of raw materials never starves the production
rate and the stock of product shipped to customers never grows so high that it
blocks the shipment rate. ????? ????? ??? ?? ????? ?????? ????? ?? ???? ????? ???? ??????? ??? ??????
.??????? ???? ??? ???? ??? ??????? ?? ???? ????? ????? ??? ???? ???? ??????
Esra Alyaheby
6.1 STOCKS, FLOWS, AND ACCUMULATION
6.1.1 Diagramming Notation for Stocks and Flows
Esra Alyaheby
6.1 STOCKS, FLOWS, AND ACCUMULATION
6.1.3 The Contribution of Stocks to Dynamics ??????? ?????? ?? ???????????
1- Stocks characterize the state of the system and provide the basis for
actions..????? ?????? ???? ?????? ????? ?????? ?????? ??????????
The stocks in a system tell decision makers where they are, providing them with
the information needed to act. ? ?????????? ????? ?????? ???????? ?? ?????? ???? ?????? ????? ???????
.??????????? ??????? ???????
For example, a firm can’t set its production schedule appropriately without knowledge
of the order backlog, the stock of inventory, the parts stocks, the labor force, and
other stocks ? ?? ???? ?????? ????? ???? ??????? ????? ??? ???? ????? ??? ????? ??????? ???? ???? ???????
? ???????? ??????? ? ?????? ????????? ? ?????? ????????? ? ?????? ????????? ???????
Esra Alyaheby
6.1 STOCKS, FLOWS, AND ACCUMULATION
6.1.3 The Contribution of Stocks to Dynamics
2- Stocks provide systems with inertia and memory..??????? ???? ????? ?? ?????? ?????????
Stocks accumulate past events..??????? ?????? ??????? ????????
The content of a stock can only change through an inflow or outflow. ??? ???? ??????
.?????? ??????? ??? ?? ???? ?????? ?? ?????? ????????
Without changes in these flows, the past accumulation into the stock persists. ??????
.? ????? ??????? ?????? ?? ????????? ???????? ?? ??? ?????????
Stocks don’t have to be tangible..??????? ?? ??? ?? ???? ???????
For example, memories and beliefs are stocks that characterize your mental states.
Your beliefs persist over time, generating inertia and continuity in your attitudes and
behavior. If you have a bad experience on an airline and never fly on that carrier
again, your belief about the low quality of their service remains even if they’ve
improved. ? ? ????? ???????? ????? ??????.? ???????? ?????????? ?? ???? ???? ?????? ????????? ???? ???? ???????
? ??? ???? ???? ????? ???? ??? ???? ????? ??? ???? ????? ??? ?????.???? ???? ?????? ?????????? ?? ?????? ???????
.? ????? ?????? ??????? ???????? ???????? ??? ?? ?? ????????? ?????? ????
Esra Alyaheby
6.1 STOCKS, FLOWS, AND ACCUMULATION
6.1.3 The Contribution of Stocks to Dynamics
3- Stocks are the source of delays..??????? ?? ???? ????????
All delays involve stocks..????? ????????? ????? ??? ???????
A delay is a process whose output lags behind its input. ???????? ?? ????? ????? ?????? ???
.??????????
The difference between the input and output accumulates in a stock of material in
process..?????? ??? ???????? ????????? ?????? ?? ????? ?? ?????? ?? ????????
When the input to a delay changes, the output lags behind and continues at the old
rate for some time..? ????? ??????? ?????? ??????? ?????? ???? ??????? ?????? ????? ??????? ??? ??????
During such adjustments, the stock accumulating the difference between input and
output changes..? ?????? ??????? ??? ????? ??? ???????? ??????????? ????? ??? ??????????
Perception delays also involve stocks though these stocks do not involve any material
flows..??? ????????? ??? ????? ?? ?? ??? ????????? ?? ????? ?? ?????? ??????
? ?????? ??????? ??????? ???
Esra Alyaheby
6.1 STOCKS, FLOWS, AND ACCUMULATION
Esra Alyaheby
Esra Alyaheby
6.1 STOCKS, FLOWS, AND ACCUMULATION
6.1.3 The Contribution of Stocks to Dynamics??????? ?????? ?? ???????????
4- Stocks decouple rates of flow and create disequilibrium dynamics. ??????? ????
.??????? ?????? ???? ???????? ??? ????????
Stocks absorb the differences between inflows and outflows, thus permitting the
inflows and outflows to a process to differ. ? ??????? ???? ?????????? ??? ???????? ??????? ?????????
.????????? ?????? ???????? ??????? ???????? ??????? ????????
In equilibrium, the total inflow to a stock equals its total outflow so the level of the
stock is unchanging. ? ??? ?????? ?????? ??? ?????? ????? ?????? ?????? ??????? ???????? ????? ??? ???? ????????
.?????? ????? ?? ??????
However, inflows and outflows usually differ because they are often governed by
different decision processes. Disequilibrium is the rule rather than the exception. ?????
? ?????? ??????? ?? ??????? ?????.? ??? ???????? ??????? ???????? ????? ???? ? ????? ???? ?????? ??????? ???? ???????? ?????
.???????????
Whenever two coupled activities are controlled by different decision makers,
involve different resources, and are subject to different random shocks, a buffer or
stock between them must exist, accumulating the difference. ?????? ??? ?????? ?? ???????
? ??? ???? ? ??????? ?????? ??????? ???????? ? ???????? ????? ???????? ????????? ?? ??? ????? ???? ????????
.? ??? ???? ??? ????? ??????? ????? ???? ???? ???? ?? ????? ???????
Esra Alyaheby
6.1 STOCKS, FLOWS, AND ACCUMULATION
6.1.3 The Contribution of Stocks to Dynamics
4- Stocks decouple rates of flow and create disequilibrium dynamics. ??????? ?????
.??????? ?????? ????? ???????? ??? ????????
As these stocks vary, information about the size of the buffer will feedback in various ways
to influence the inflows and outflows. ? ??? ????????? ???????? ???? ???????? ????? ??????? ??? ??????????
?
.??????? ??? ???? ???????? ???? ?????? ??????? ??? ???????? ??????? ?????????
Often, but not always, these feedbacks will operate to bring the stock into balance. ????
.? ????? ??? ????????? ?????? ??????? ?? ????????? ? ???? ??? ??? ????? ????? ?? ????????
Whether and how equilibrium is achieved cannot be assumed but is an emergent
property of the whole system as its many feedback loops interact simultaneously. ??? ?????
? ????? ????? ????? ?????? ?????? ??? ?????? ????? ??????? ????????? ??????? ?? ??? ??? ??? ????? ??????? ?????? ???????
.???????? ?????? ?? ?? ??? ?????
Understanding the nature and stability of these dynamics is often the purpose of a
system dynamics model..??????? ?? ???? ??? ????? ???????? ?????? ?????????? ?? ??? ????? ???????? ???????
Esra Alyaheby
6.2 Identifying Stocks and Flows ?????? ??????????
???????????
The distinction between stocks and flows is recognized in many disciplines ?????
.???????? ??? ?????? ????????? ?? ?????? ?? ?????????
.
Esra Alyaheby
6.2 Identifying Stocks and Flows
How can you tell which concepts are stocks and which are flows?
???? ????? ????? ?? ???????? ?? ?????? ??? ??????????
o Stocks are quantities of material or other accumulations. They are the
states of the system..? ?? ??? ???????.?????????? ?? ????? ?????? ?? ????????? ???????
The flows are the rates at which these system states change. ????????? ?? ?????????
.o????? ????? ??? ????? ?????? ????
6.2.1 Units of Measure in Stock and Flow Networks ?????? ?????? ???
?????? ?????? ????????
The units of measure can help you distinguish stocks from flows ??????
.??? ?????? ????? ?????? ??? ??????? ??? ?????? ??????????
o Stocks are usually a quantity such as widgets of inventory or people
.
employed..??????? ???? ?? ???? ???? ??? ???????? ?? ??????? ?? ??????? ?????????
The
o associated flows must be measured in the same units per time period.
For example, the rate at which widgets are added per week to inventory, the
hiring rate in people per month. ? ????.???? ???? ???????? ???????? ???? ??????? ??? ???? ??????
? ????? ??????? ?? ????????? ? ???? ????? ????? ????? ???????? ???????? ??? ?????…
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