Introduction to System Dynamics

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Transcript Introduction to System Dynamics

Tools for Systems Thinking
and Modeling
Dynamics: Graphs over time
Structure: Causal-loop Diagrams
Stocks and Flows
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A Systems Perspective
Reactive
Events and Decisions
Adaptive
Patterns of Behavior
Generative
System Structure
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“Distancing...”
A systems view stands back just far enough to...
• Deliberately blur discrete events into patterns of
behavior
• Deliberately move from a focus on individual
decisions to a focus on policy structure
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The Systems Perspective
Reactive
Events and Decisions
Adaptive
Patterns of Behavior
Generative
System Structure
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Dynamics
• Define problems in terms of graphs over time.
• Graph important variables
• Graph historical data
• Graph anticipated dynamics
• Graph preferred dynamics
• Use these to focus systems thinking and modeling
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Unemployment (%)
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New York City Population, 1900-2000
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Unemployment & Welfare
in Dutchess County, NY
Welfare roll
Unemployment
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Per Capita Residential Energy Use
(USA Today 3/23/09, citing Census Bureau and U.S. Energy Information Administration)
5000
4000
3000
2000
1000
0
1950
1960
1970
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1980
1990
2000
2010
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Strive for Insights
Here’s where we went wrong...
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The Systems Perspective
Reactive
Events and Decisions
Adaptive
Patterns of Behavior
Generative
System Structure
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Systems Structure
•
•
•
•
•
•
•
Accumulations (populations, resources…)
Causal structure: “feedback” loops
Delays
Perceptions (a kind of accumulation)
Pressures
Affects, emotions, (ir)rationalities
Policies governing decisions
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Causal Loop Diagrams
Crime rate
• Causal mapping is a
powerful tool for
representing structure in
complex systems.
• Arrows indicate causal
influence.
Quality of city life
Neighborhood
health clinics
Inmigration
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Retail outlets
Population
Absentee
landlords
Outmigration
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Polarities of Causal Links
• Positive and negative
signs show the direction of
causality:
+ ... “direct” relation
– …“inverse” relation
Crime rate
+
–
Quality of city life
–
+
Neighborhood
health clinics
Inmigration
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Retail outlets
+
Population
Absentee
landlords
–
Outmigration
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Definitions of Link Polarities
A adds to B, or
∆A leads to ∆B in the same
direction
C subtracts from D, or
∆C leads to ∆D in the opposite
direction
All words phrases are expressed as quantities that have a clear sense of
increase or decrease.
No verbs — the action is in the arrows.
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Simple test for link polarity
An increase in A makes B
higher than it would have
been without the change.
An increase in C makes D
lower than it would have
been without the change.
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Examples
Lawyers
Outmigration
+ Court cases
–
Population
More lawyers mean more
litigation; fewer lawyers,
less litigation
Emigration subtracts from
population: An increase in
emigration means less (a
decrease means more) than
we’d have without the change
“Ceteris paribus”...
All other influences held constant as we assign polarities.
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Exercises
Reglatory delays in
the planning process
Citizen ownership of
the outcomes
Citizen participation in
the planning process
Anxiety
Productivity
Ratiation
of heat away
d
from the earth
Concentration of
green house gases in
atmosphere
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Global average
temperature
18
Feedback Loops
A feedback loop exists when decisions change the state
of the system, changing the conditions and information
that influence future decisions.
Action
State of
the system
Decisions
Perceptions
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The Joy of Feedback
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The Joy of Feedback
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No — it’s more like the life cycle of the famous
scientist
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Examples of Feedback Loops
Solvency of
a Bank
Deposits
Withdrawals
Perceived solvency
of the Bank
Quality of
communication
Trust
Risk taking
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Quality of
communication
between teams
Quality of
communication
within teams
Teamwork
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Two kinds of feedback loops
• Reinforcing loops
• Balancing loops
• Growth producing
• Counteracting
• Destabilizing
• Goal seeking
• Accelerating
• Stabilizing
• Positive: an even number of –’s
• Negative: an odd number of –’s
• Symbolized by
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• Symbolized by
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Examples of Reinforcing Loops
Performance
Births per
year
+
+
Population
+
+
Motivation
+
Number of private
businesses
Attractiveness
for business
+
+
Expected profitability
of business
–
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Tax rate
Tax base
–
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Isolate and Identify Link Polarity
Attractiveness for
business
Number of private
businesses
Expected
profitability of
business
Tax base
Tax Rate
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Isolate and Identify Link Polarity
(Always trace an Increase)
Attractiveness for
business
+
Number of private
businesses
Expected
profitability of
business
Tax base
Tax Rate
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Isolate and Identify Link Polarity
(Always trace an Increase)
Attractiveness for
business
Number of private
businesses
+
Expected
profitability of
business
Tax base
Tax Rate
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Isolate and Identify Link Polarity
(Always trace an Increase)
Attractiveness for
business
Number of private
businesses
Expected
profitability of
business
Tax base
Tax Rate
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–
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Isolate and Identify Link Polarity
(Always trace an Increase)
Attractiveness for
business
Number of private
businesses
Expected
profitability of
business
–
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Tax base
Tax Rate
30
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Isolate and Identify Link Polarity
(Always trace an Increase)
Attractiveness for
business
Number of private
businesses
+
Expected
profitability of
business
Tax base
Tax Rate
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Completed Loop
(Now tell the story)
Attractiveness for
business
+
Number of private
businesses
+
+
(R)
Expected
profitability of
business
–
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Tax base
Tax Rate
–
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Typical Reinforcing Loop Behaviors
20,000
15,000
Population and Births Loop
10,000
5,000
10,000
0
0
25
50
75
100
9,000
8,000
Businesses and Taxes Loop
7,000
6,000
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25
50
75
100
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World Population (billions)
8
6
4
2
0
1500
1600
1700
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1800
1900
2000
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Self-reinforcing processes in world population
growth
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Reinforcing Feedback in the Newspaper
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Reinforcing Feedback in the Newspaper
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Reinforcing Feedback in the Newspaper
Garfield’s (R) Jon’s
happiness
suspicion
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Examples of Balancing or Counteracting Loops
+
Water
in glass
Desired amount of
water in glass
+
Pouring rate
+
Population
–
-
Fraction
filled
–
Gypsie moth
net growth
–
Wasps
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Outmigration
+ Gypsie moths
+
+
Wasp net
growth
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Typical Counteracting Loop Behaviors
20
Filling a Glass
15
Predator-prey interactions
10
5
0
010,000
10
20
30
40
7,500
•0
5,000
•15
•22.5
•30
Population and
emigration
2,500
0
•7.5
0
25
50
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75
100
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Tips for Determining Link and Loop Polarities
• For each link, determine the effect of an increase in the variable
at the tail of the arrow:
• If the variable at the head increases, assign a plus.
• If the variable at the head decreases, assign a minus.
• For each loop, count the number of negative signs:
• An even number of negative links is a reinforcing (R) loop.
• An odd number of negative links is a balancing (B) loop.
• Most important: For each loop, tell a self-reinforcing or
balancing/counteracting story, and check that the story
matches the loop polarity.
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More Serious Example from the Newspaper:
Recall the graph of per capita energy use
5000
4000
3000
2000
1000
0
1950 1960 1970 1980 1990 2000 2010
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Presentation Dynamics
Clarity of
presentation
Audience
understanding
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Clarifying
questions
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Presentation Dynamics
- Which loop dominates?
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Stocks and Flows
Stocks are accumulations.
• Stocks are increased by inflows and decreased by outflows.
• When a link means “add” or “subtract” we have a stock-andflow structure.
• Example: Inventory
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Stock and Flow Diagramming Conventions
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A Stock and Flow Example
Explicit stocks and flows:
The corresponding causal-loop diagram:
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Human Activity, CO2, and Global Temperature
Thought
experiment:
Capital
stock
CO2 annual
production
Atmospheric
CO2
Uptake of
atmospheric CO2
Economic
activity
capital
investment
incoming solar
heat energy
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Global heat
energy
outgoing global
heat energy
48
Structure and Dynamics of Terrorist Cells
Peripheral
support
(R)
Suppression
activities
(R)
(R)
(R)
Zeal
Funding
New
(R) Terrorist
recruits
group
(R)
Martyrs to
the cause
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Terrorist
actions
(B)
(B) Losses
(R)
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Balancing
Loops
Balancing
or
in Tobacco Loops
Counteracting
Prevalence
Researchers
awarness of
tobacco health
risk
+
Smokers
+
Public
awareness of
tobacco health
risk
+
Three views
People quitting
smoking
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Tax revenues
from smokers
Balancing
Loops
Balancing
or
in Tobacco Loops
Counteracting
Prevalence
+
+ Govt funding of
tobacco control
Researchers
awarness of
tobacco health
risk
Funding for tobacco
health research
+
+
Smokers
Public
awareness of
tobacco health
risk
-
People quitting
smoking
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+
Funding for tobacco
control programs
+
+
+
Three views
+Gov. income
+
Tobacco control
programs
+
+
Health insurers
coverage of tobacco
quitting costs
51
Loops and Constituencies
Pro-tobacco
contituencies
+ +
+
Anti-tobacco
constituencies
Govt awareness of
tobacco health risk
+
+
+
+
+
+
Researchers awarness
of tobacco health risk
Health care costs
+
Funding for tobacco
health research
+
+
+
+
Smokers
Tobacco
revenues
+
+
Trend in tobacco
company revenues
+
Tobacco growers
+
Tobacco control
programs
+
People starting smoking
+
People quitting
+
smoking
-
+
+
Public awareness of
tobacco health risk
-
+
Funding for tobacco
control programs
Smoking as a
social norm
-
+
+
Tobacco marketing
activities
-
+
+
Tobacco products
availability
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Pressure on tobacco
companies to reduce
marketing activities
+
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There are a lot …
Perceived importance of
other health programs
<Anti-tobacco
litigation>
-
+
+
+Tobacco taxes
Govt awareness
of tobacco
health risk
+
+
Pro-tobacco
contituencies
++
+
+
+
Trend in tobacco
company revenues
+
Tobacco
production
capacity
+
Smoking as a
social norm
+
Public
awareness of
tobacco health
risk
+
+
+
People starting
smoking
+
People quitting
+
smoking
+
-
+
Funding for tobacco
control programs
+
+
-
+
Tobacco control
programs
+
+
Health insurers
coverage of tobacco
quitting costs
+
Tobacco marketing
activities -
+Gov. income
+
Funding for tobacco
health research
+
+
Smokers
+
Tax revenues
from smokers
+ Govt funding of
tobacco control
Researchers
awarness of
tobacco health
risk
+
+
Tobacco revenues
+
Anti-tobacco
constituencies
+
+
Health care costs
Anti-tobacco
litigation
Anti-tobacco
legislation
<Perceived importance
of other health
programs>
- Govt willingness to +
act against tobacco
Tobacco products
+
availability
xxx
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Pressure on tobacco
companies to reduce
marketing activities
53
Policy Resistance of Complex Systems
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Policy Resistance of Complex Systems
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Policy Resistance of Complex Systems
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Policy Resistance in Complex Systems
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