Quiz
Soeavam007
02.2StockFlowDiagrams.pptxTools for Describing System Structure
Week 2
“If a factory is torn down but the rationality which produced it is left standing, then that rationality will simply produce another factory. If a revolution destroys a government, but the systematic patterns of thought that produced that government are left intact, then those patterns will repeat themselves… There’s so much talk about the system. And so little understanding.”
Robert Pirsig, Zen and the Art of Motorcycle Maintenance
(An Excerpt from Meadows, Thinking in Systems)
System Dynamics is a World View and a Tool.
Key Characteristics:
Systems Perspectives
Feedback Thinking
Stocks and Flows
Endogenous Point of View
Use of Computer Simulation Modeling
Tools for Structural Thinking
Language to Describe the System Structure
Word-and-arrow relationships
Feedback loops (CLDs)
Stock-and-flows
Tools for Linking System Structure to its Behaviors
Computer simulation modeling
Overcomes bounded rationality
Incorporates Empirical Data
Allows Scenario Testing
Stocks and Flows
Inflow
Outflow
Stock
A stock increases when Inflow > Outflow
A stock decreases when Inflow < Outflow
A stock is unchanged when Inflow = Outflow
A stock accumulates things : people, dollars, staff, morale…
6
Inflows add to stocks
Outflows deplete stocks
Example:
7
Example:
Stock Outflow
-
Units of Measure
The choice of time unit for the flows (e.g. days, weeks, years) is arbitrary but must be consistent within a single model.
Figure 6-3
Stocks change only through their rates.
Figure 6-8
How many people are in the store at each minute?
11
Use example from people leaving and entering store, or bathtub example from OBHDP article
Graph of previous slide
People in the store
Minutes
When did the most people enter the store?
When did the most people leave the store?
When are the fewest people in the store?
When are the most people in the store?
13
When did the most people enter the store?
When did the most people leave the store?
When are the fewest people in the store?
When are the most people in the store?
Table 7-1
Stocks accumulate (integrate) their net flows.
Table 7-1
The slope of the stock trajectory is the net rate of change (derivative) of the stock.
Example of Graphical Integration
Figure 7-2
Stocks and Flows
Why are they so important?
Global Warming Simulation
For the past 425,000 years, the amount of CO2 in the atmosphere fluctuated between 175 parts per million (ppm) and 300 ppm. Equilibrium of CO2 inflow and outflow
In 2007 (present), CO2 concentration is approximately 380 ppm. Drastic increase in CO2 inflow
Environmental scientists have identified that to avoid the most significant damage to the Earth’s ecosystem, the maximum level of CO2 concentration should be no higher than 450 ppm.
What should we do?
Global Warming Simulation
What should we do?
Allow Increasing CO2 Emissions
No regulation
Business as usual
Level off CO2 Emissions
Kyoto Protocol
Lower greenhouse gas emissions to 5.2% lower than 1990 levels by 2012
Reduce CO2 Emissions
Al Gore, David Stern
Reduce CO2 emission by 58% of the 2007 level by 2070
Play Simulation
http://www.planetseed.com/relatedarticle/climate-challenge-our-choices
Stock
Inflow
Outflow
Inventory
Production
Shipments
Savings
Interest
Earned
Withdrawals
Interest Rate
(R)
(B)
Change in Expected
Order Rate (widgets/week/week)
Expected Customer
Orders (widgets/week)
Birth Rate (people/year)
Population (people)
Death Rate (people/year)
Borrowing ($/year)
Debt ($)
Repayment ($/year)
Rate of Price Change
($/unit/year)
Product Price
($/unit)
Hire Rate (people/year)
Quit Rate (people/year)
Layoff Rate (people/year)
Employees (people)
Retirement Rate (people/year)
Change in
Expected
Order Rate
(widgets/week/week)
Expected
Customer
Orders
(widgets/week)
Birth Rate
(people/year)
Population
(people)
Death Rate
(people/year)
Borrowing
($/year)
Debt
($)
Repayment
($/year)
Rate of Price
Change
($/unit/year)
Product
Price
($/unit)
Hire Rate
(people/year)
Quit Rate
(people/year)
Layoff Rate
(people/year)
Employees
(people)
Retirement Rate
(people/year)
Service Staff
B
Productivity
Workweek
Customer Arrival Rate
--
+
-
+
Customer Departure
Rate
B
Productivity
Workweek
+ ++
+
+
Customers Waiting for
Service
B
B
Customer Arrival Rate
Customer Departure
Rate
Service Staff
Correct
Incorrect
Customers Waiting for
Service
Service
Staff
B
Productivity
Workweek
Customer
Arrival
Rate
-
-
+
-
+
Customer
Departure
Rate
B
Productivity
Workweek
+
+
+
+
+
Customers
Waiting for
Service
B
B
Customer
Arrival
Rate
Customer
Departure
Rate
Service
Staff
Correct
Incorrect
Customers
Waiting for
Service
People entering and leaving a store
0
2
4
6
8
10
12
12345
Minute
People
Entering
Leaving
Chart2
| 10 | 5 |
| 10 | 5 |
| 10 | 5 |
| 10 | 5 |
| 10 | 5 |
Sheet1
| Entering | Leaving | |
| 10 | 5 | |
| 10 | 5 | |
| 10 | 5 | |
| 10 | 5 | |
| 10 | 5 | |
| Entering | Leaving | |
| 15 | 10 | |
| 25 | 0 | |
| 15 | 12 | |
| 30 | 20 | |
| 27 | 22 |
Sheet1
Sheet2
Sheet3
People entering and leaving a store
0
5
10
15
20
25
30
35
12345
Minute
People
Entering
Leaving
Chart3
| 15 | 10 |
| 25 | 0 |
| 15 | 12 |
| 30 | 20 |
| 27 | 22 |
Sheet1
| Entering | Leaving | |
| 10 | 5 | |
| 10 | 5 | |
| 10 | 5 | |
| 10 | 5 | |
| 10 | 5 | |
| Entering | Leaving | |
| 15 | 10 | |
| 25 | 0 | |
| 15 | 12 | |
| 30 | 20 | |
| 27 | 22 |
Sheet1
Sheet2
Sheet3
S to
c k
(u n
it s )
400
300
200
100
0
Time (seconds)
N e t
F lo
w (u
n it
s /s
e c o
n d
) 20
10
0
10 200 30
S
t
o
c
k
(
u
n
i
t
s
)
400
300
200
100
0
Time (seconds)
N
e
t
F
l
o
w
(
u
n
i
t
s
/
s
e
c
o
n
d
)
20
10
0
102003 0
