sd slides 504rev05
TRANSCRIPT
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany 1
Tools for Systems Thinkingand Modeling
Dynamics: Graphs over time
Structure: Causal-loop Diagrams
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany 2
Events and Decisions
Patterns of Behavior
System Structure
Reactive
Adaptive
Generative
A Systems Perspective
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany 3
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
Distancing...
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany 4
Events and Decisions
Patterns of Behavior
System Structure
Reactive
Adaptive
Generative
The Systems Perspective
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany 5
Dynamic Thinking
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 andmodeling
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany 6
Unemployment(%)
11
10
9
8
7
6
5
4
3
2
19 48 19 58 19 68 19 78 19 88 19 98
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany 7
New York City Populations
8000000
7000000
6000000
5000000
4000000
3000000
2000000
1000000
01900 1910 1920 1930 1940 1950 1960 1970 1980 1990
Bronx
Brooklyn
ManhattanQueens
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany 8
Percentages of high school gradscompleting college, by ethnicity
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
1971
1973
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
White
BlackHispanic
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany 9
Unemployment & Welfare in DutchessCounty, NY
Unemployment
Welfare roll
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany 10
New York State K-12 Public EducationExpenditures per Pupil
$0
$2,000
$4,000
$6,000
$8,000
$10,000
$12,000
$14,000
1965 1977 1981 1985 1989 1993 1997 2001
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany 11
Heres where we went wrong...
Strive for Dynamic Insights
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany 12
So youre getting moreeducation
Income
Time
Your Historical Income
Your Hoped For Income
Your Feared Income
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany 13
Events and Decisions
Patterns of Behavior
System Structure
Reactive
Adaptive
Generative
The Systems Perspective
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany 14
Accumulations
AKA Stocks
Accumulations are both tangible and
intangible
Resources, populations
Preferences, feelings, pressures, perceptions
Causal structure: feedback loops Delays
Policies governing decisions
System Structure
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Causal Diagrams
Causal mapping is a
powerful tool for
representing structurein complex systems.
Arrows indicate
causalinfluence.
Student achievement
in school
Student native
abilityFamily stress
Underage drug and
alcohol use in the
community
Teacher talent and
resources
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany 16
Polarities of Causal Links
Positive and negative
signs show the
direction of causality:
+ = direct relation
= inverse relation
Student achievement
in school
Student native
abilityFamily stress
Underage drug and
alcohol use in the
community
Teacher talent and
resources
+
+
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany 17
Definitions of Link Polarities
All words phrases are expressed as quantitiesthat have aclear sense of increase or decrease.
No verbsthe action is in the arrows.
An increasein A makes B
greater than it would have
been without the change; a
decreasein A makes B less.
An increasein C makes D less
than it would have been without
the change; a decreasein C
makes D greater.
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany 18
Examples
Ceteris paribus...
All other influences held constant as we assign polarities.
More teachers mean smaller
classes; smaller classes mean
morelearning. Fewer
teachers would mean largerclasses and less learning.
Higher test scores mean
moresupport; lower
scores, less support.
Teachers Class SizeStudent
Learning
Test ScoresParental Support
for SchoolAdministration
+
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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.
Decisions State of
the system
Action
Perceptions
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The Joy of Feedback
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The Joy of Feedback
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Noits more like the life cycle ofthe famous scientist
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany 23
Examples of Feedback Loops
Quality of
communication
Quality of
communicationbetween teams
Quality of
communicationwithin teams
TeamworkRisk taking
Trust
Student effort
Teacher morale
Teachereffort
Student
achievement
+ +
++
+
+
+
+
+ +
+
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Two kinds of feedback loops
Reinforcing loops
self-reinforcing
growth producing
destabilizing
accelerating
even number ofs
Symbolized by
Counteracting loops
balancing
goal seeking
stabilizing
compensating
odd number ofs
Symbolized by
R + C B
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany 25
Examples of Reinforcing Loops
Number of private
businesses
Tax base
Tax rate
Expected profitabilityof business
Attractiveness
for business
PopulationBirths per
year
Performance
Motivation
+
+
+
+
++
+
--
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Reinforcing Feedback in theNewspaper
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Reinforcing Feedback in theNewspaper
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany 28
Reinforcing Feedback in theNewspaper
Garfields
happinessJons
suspicion(R)
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany29
Examples of Balancing Loops
Tax revenues
Desired spending on
education
Education revenues
from the lottery
Total education
revenues
Pressure to raise tax
revenues for education
Pressure to
raise taxes
Share of tax
revenues to education
+
+
+
+ ++
+
-
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany30
What are these loops,and what stories do they tell?
Network
complexity
Ease of communication
and collaboration
Educational
improvement
Number of agencies
involved
Desired educational
improvement
Total schoolimprovement effort
Average agency
effectiveness
Student effort
Teacher morale
Teachereffort
Student
achievement
+
++
+
+
+-
-
+
+ +
+
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany31
Tips for Determining Link and LoopPolarities
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-reinforcingor balancing/counteracting story, and check that the
story matches the loop polarity.
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany32
Linking Feedback Structure and
Dynamic Behavior
Linking causal loop thinking with
graphs over time
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany33
World Population (billions)
8
6
4
2
0
1500 1600 1700 1800 1900 2000
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany34
Self-reinforcing processes in worldpopulation growth
PopulationBirths
per yearDeathsper year
AveragelifespanCapital
accumulation
Knowledgeaccumulation
R
R
R
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany35
Typical Reinforcing Loop Behavior
Number of private
businesses
Tax base
Tax rate
Expected profitability
of business
Attractiveness
for business
10,000
9,000
8,000
7,000
6,000
0 25 50 75 100
Businesses and taxes loopin a reinforcing decline
Lets assume a tax increase.
What will the graph look like?
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System Dynamics Group, Rockefeller College of Public Affairs and Policy, University at Albany36
Typical Counteracting Loop Behavior
Desired amount of
water in glassWater
in glass
Pouring rate
Fraction
filled
20
15
10
5
00 10 20 30 40
Filling a Glass
+
+
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Typical Counteracting LoopBehavior
Gypsie mothsGypsie mothnet growth
Wasps Wasp netgrowth
0 7.5 15 22.5 30
Predator-prey interactions
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System Dynamics Group Rockefeller College of Public Affairs and Policy University at Albany38
Summary of Structure andBehavior
Reinforcing loops are responsible for acceleratinggrowth, accelerating decline, or traps due to inertia
Counteracting loops are much more prevalent but
often less obvious; they balance and adapt
Counteracting loops can compensate for policyinitiatives and defeat intended effects
Combinations of reinforcing and counteractingloops are responsible for all the complex dynamicswe create in our complex systems.