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
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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
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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
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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
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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
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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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