SYSTEM DYNAMICS and Systems Thinking - PowerPoint PPT Presentation

View by Category
About This Presentation
Title:

SYSTEM DYNAMICS and Systems Thinking

Description:

... The Sector Approach to SD model formulation Begin by identifying the sectors A sector is all the structure ... Enemy is out there ... situation unfolding in ... – PowerPoint PPT presentation

Number of Views:451
Avg rating:3.0/5.0
Slides: 147
Provided by: Jam143
Learn more at: http://burns.ba.ttu.edu
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: SYSTEM DYNAMICS and Systems Thinking


1
SYSTEM DYNAMICS and Systems Thinking
  • developed by James R. Burns

2
Why????
  • Because of the frenetic increases in
    complexitysociety is becoming increasing complex
    and we need tools to cope with it
  • This is certainly true in the IS/IT arena as well

3
Coping tools
  • Causal modeling
  • Simulation
  • Discrete stochastic (Promodel)
  • Continuous deterministic (Vensim)

4
System Dynamics and Vensim
  • A tutorial on Vensim is provided at the end of
    Chapter 6 in your copy packet, beginning on page
    35 of that chapter

5
Dynamic problems appropriate for Vensim (rather
than promodel)
  • There is change over time
  • The changing character of the situation IS THE
    PROBLEM
  • The problem should be studied in aggregates
  • The problem does not have a significant
    stochastic component or complexion to it

6
Start with descriptions of the following
  • PURPOSE
  • PERSPECTIVE
  • PROBLEM
  • MODE

7
What are we doing here????
  • Attempting to characterize, cope with and
    understand complexity
  • Especially DYNAMIC complexity
  • Inventing a physics for a systems or processes
    for which there exists no physics
  • You get to become a Newton, a Liebnitz, a
    Galileo, an Einstein, a .

8
Steps
  • Be problem-driven
  • Interview people familiar with the problem
  • Gather verbal descriptions of the problem
  • Formulate a list of variables
  • Develop a causal loop diagram
  • Develop a stock-and-flow diagram
  • Create a working simulation in VENSIM

9
How many of you have used a model to solve a
problem or make a decision?
  • Youve been through this drill before!
  • All of you have.all of the time!

10
Problem
Problem
SD Model
Mental Model
Mental Model
Decision
Decision
Action
Action
11
Uses to which these models can be put
  • What IF experimentshands on experimentation
  • Decision making
  • Planning
  • Problem solving
  • Creativity
  • Out of the box thinking
  • Hypothesis testing
  • Finding leverage points, points of intervention
  • LEARNING

12
Some notation--
  • CLD Causal Loop Diagram
  • SFD Stock-and-Flow Diagram
  • BOT Behavior Over Time Chart

13
A CLD of the US energy system
14
A roadmap for the U.S. energy system
15
BOTBehavior Over Time chart
  • For US energy market

16
US energy consumption, by source, 1850-2000.
(Units quadrillion BTU)
(Source Energy use in the United States.
Available from http//en.wikipedia.org/wiki/Energy
_use_in_the_United_States)
17
From CLD to SFD
  • System dynamics model
  • Causal loop diagram
  • Stock-and-flow diagram

vs.
Mathematical representation for stock
18
Extraordinary Organizations
  • Are those that engage peoples commitment and
    capacity to learn at all levels in the
    organization
  • Will recognize that the only truly sustainable
    competitive advantage is the rate at which
    organizations learn
  • Nothing compares to the exhilaration that comes
    from working within learning orgs.

19
Ordinary Organizations.
  • Learn slowly if at all
  • Characterize an organization that you are aware
    of..

20
Disciplines of the Learning Organization
  • Personal Mastery
  • Mental Models
  • Shared Vision
  • Team (Organizational) Learning
  • Systems Thinking

21
Personal Mastery
  • Continually clarifying and deepening our personal
    vision
  • Getting better at what we do best

22
Mental Models
  • Deeply engrained assumptions, generalizations

23
Shared Vision
  • Where there is genuine vision, people excel
  • Where there is no vision the people perish

24
Team Learning
  • The synergy of teams is the ultimate exhilaration
  • Some people, having experienced it once, spend
    the rest of their lives looking for it

25
Systems Thinking
  • All human endeavors are systems

26
The Fifth Discipline
  • IS, OF COURSE, SYSTEMS THINKING
  • Subsumes and permeates all of the other
    disciplines
  • By enhancing the other disciplines, it
    continually reminds us that the whole can exceed
    the sum of its parts
  • But ST also needs the other four disciplines to
    realize its full potential

27
Metanoia--A shift of Mind
  • The recreation of ourselves through learning
  • Becoming able to do something we never were able
    to do
  • Re-perceiving the world and our relation to it
  • Extending our capacity to create
  • There is within each one of us a deep hunger for
    this type of learning

28
Putting the Ideas into Practice
  • SENGE The greatest societal problem facing us
    today is the increased complexity of our systems
  • FORRESTER Systems are counterintuitive.
    Consequently, naïve policy makers implement
    policies that have just the opposite of their
    intended effect

29
Senges Metanoia
  • Originally, he was interested only in public
    sector problems
  • But then corporate leaders came to him for help
  • These were thoughtful people, deeply aware of the
    inadequacies of their own organizations
  • All shared a commitment and capacity to innovate
    that was lacking in the public sector

30
Who were these people???
  • William OBrien of Hanover Insurance
  • Edward Simon from Herman Miller
  • Ray Stata, CEO of Analog Devices
  • Trammel Crow
  • Arie De Geus of Shell Oil Co
  • Leaders from Apple, Ford, Polaroid,
  • 4000 Managers who attended the Innovations
    Associates workshops over eleven years

31
I am my Position
  • We are trained to be loyal to our jobsso much so
    that we confuse our job with our personal
    identity.
  • Most people see themselves within a system over
    which they can exercise little control
  • There is a kind of myopia in American
    organizations that causes individual workers to
    focus only on their small part rather than on the
    larger system as a whole
  • APICS is trying to address this problem
  • We need to see ourselves in the context of the
    larger system

32
The Enemy is out There
  • Generally, we tend to see the problem as outside
    us
  • no one can catch a ball in that darn field
  • Thou shalt always find an external agent to
    blame
  • Marketing blames manufacturingquality is poor,
    due dates are missed, etc.
  • Manufacturing blames Engineering
  • Engineering blames Marketing

33
The Enemy is out there is actually
  • A byproduct of I am my position
  • Because of the non-systemic ways of looking at
    the world that it fosters
  • When we focus only on our position, we do not see
    how our actions extend beyond the boundary of
    that position
  • When those actions have consequences that go
    beyond our position, they come back to hurt us

34
The Enemy is out there manifests itself with
statements like..
  • the Japanese are killing us
  • The labor unions are killing us
  • The government regulators are killing us
  • But this is always an incomplete story that fails
    to recognize that out there and in here are
    parts of the SAME SYSTEM

35
The Illusion of Taking Charge
  • Being proactive is in vogue
  • Just ask Steven Covey
  • This means face up to difficult problems, stop
    waiting for someone else to do something, solve
    problems before they grow into crises, etc.
  • We have a hooked on heroics cultureone that
    always looks for leadership from the top

36
The Illusion of Taking Charge
  • All too often pro-activeness is re-activeness in
    disguise
  • True pro-activeness comes from seeing how we
    contribute to our own problems

37
The Fixation of Events
  • We are conditioned to see life as a sequence of
    events
  • The situation unfolding in Kashmir is viewed as a
    sequence of escalating events
  • The situation in Israel/Palestine again is seen
    as a situation involving events which can be used
    to justify the position of either side

38
The Fixation of Events
  • The media reinforces the fixation on events
  • That is what they report
  • It is part of our programming
  • Distracts us from seeing the longer term patterns
    of change that underlie events and from
    understanding the causes that underlie the
    patterns

39
Today, the primary threats to our survival\
  • Stem not from events but from slow gradual
    processes
  • The environment
  • The erosion of public education
  • Generative thinking cannot be sustained if people
    are focused on events

40
The Parable of the Boiled Frog
  • What is it???

41
The Delusion of Learning from Experience
  • We learn from taking an action and observing the
    consequences of that actions
  • What happens when we cannot observe the
    consequences of our action?
  • We all have a learning horizona span in time and
    space within which we assess our effectiveness
  • We learn best from experience but we never
    directly experience the consequences of many of
    our most important decisions

42
The Delusion of Learning from Experience
  • Most people have short memories
  • If cycles last longer than a year or two, they
    are particularly hard to see and thus learn from
  • To reduce the breadth of impact organizations are
    decomposed into components
  • But the departments create stovepipes that reduce
    the observability of complex issues that cross
    functional boundaries.

43
Reinforcing loops vs. Balancing loops
44
Stock involving both reinforcing and balancing
loops
45
Stock-and-Flow Diagram
Mathematical representation for Population
INTEGRAL ( ) function in the VENSIM
model Population INTEGRAL (Births Deaths,
Population(t0))
46
The Methodology once problem is identified
  • Find substance
  • Delineate CLDs, BOT charts
  • Submit these for outside scrutiny
  • Delineate SFD
  • Implement simulation in VENSIM
  • Submit for outside scrutiny
  • Utilize model for policy experimentation

47
Find substance
  • Written material
  • Books
  • Articles
  • Policy and procedure manuals
  • Peoples heads
  • Order of magnitude more here
  • Must conduct interviews, build CLDs, show them
    to the interviewee to capture this

48
Delineate CLDs, BOTs
  • CLD Causal Loop Diagram
  • BOT Behavior-over-Time chart
  • Collect info on the problem
  • List variables on post-it notes
  • Describe causality using a CLD
  • Describe behavior using a BOT diagram

49
Submit these for outside scrutiny
  • We simply must get someone qualified to assess
    the substance of the model

50
Delineate SFD
  • SFD Stock and Flow Diagram
  • Translate CLD into SFD

51
What are stocks and flows??
  • A way to characterize systems as stocks and flows
    between stocks
  • Stocks are variables that accumulate the affects
    of other variables
  • Rates are variables that control the flows of
    material into and out of stocks
  • Auxiliaries are variables that modify information
    as it is passed from stocks to rates

52
Stock and Flow Notation--Quantities
  • STOCK
  • RATE
  • Auxiliary

53
Stock and Flow Notation--Quantities
  • Input/Parameter/Lookup
  • Have no edges directed toward them
  • Output
  • Have no edges directed away from them

54
Inputs and Outputs
  • Inputs
  • Parameters
  • Lookups
  • Inputs are controllable quantities
  • Parameters are environmentally defined quantities
    over which the identified manager cannot exercise
    any control
  • Lookups are TABLES used to modify information as
    it is passed along
  • Outputs
  • Have no edges directed away from them

55
Stock and Flow Notation--edges
  • Information
  • Flow

56
Some rules
  • There are two types of causal links in causal
    models
  • Information
  • Flow
  • Information proceeds from stocks and
    parameters/inputs toward rates where it is used
    to control flows
  • Flow edges proceed from rates to states (stocks)
    in the causal diagram always

57
Robust Loops
  • In any loop involving a pair of quantities/edges,
  • one quantity must be a rate
  • the other a state or stock,
  • one edge must be a flow edge
  • the other an information edge

58
CONSISTENCY
  • All of the edges directed toward a quantity are
    of the same type
  • All of the edges directed away from a quantity
    are of the same type

59
Population problem
  • Population has grown in the last 102 years from
    1.65 billion persons to 6.2 billion persons on
    planet earth today
  • WHAT IS THE CARRYING CAPACITY OF THE PLANET??
  • Depends on what material standard living you
    assume
  • Birth rates, due to improved health, and death
    rates are lower due again to improved health
  • Corresponding to each, there is a normal
    condition

60
List VARIABLES
  • Population
  • Birth rate
  • Death rate
  • Death rate normal
  • Birth rate normal

61
Draw Causal Loop Diagram
62
Converting to a STOCK AND FLOW Diagram
  • What is a STOCK?
  • What is a FLOW?
  • What is a RATE?
  • What is a parameter?

63
Convert CLD to SFD
64
Determine equations
  • BRN .04
  • DRN .028
  • BR BRNP
  • DR DRNP
  • P(t dt) p(t) dt(BR DR)

65
The Sector Approach to SD model formulation--
  • Begin by identifying the sectors
  • A sector is all the structure associated with a
    single flow
  • There could be several states in a single sector

66
The sector Approach, Continued
  • Determine the within-sector structure
  • Reuse existing molecules where possible
  • Determine the between-sector information
    infrastructure
  • There are no flows and therefore no stocks or
    rates here

67
A Single-sector Exponential goal-seeking Model
  • Sonya Magnova is a television retailer who wishes
    to maintain a desired inventory of DI television
    sets so that she doesnt have to sell her
    demonstrator and show models. Sonyas ordering
    policy is quite simple--adjust actual inventory I
    toward desired inventory DI so as to force these
    to conform as closely as possible. The initial
    inventory is Io. The time required for ordered
    inventory to be received is AT.

68
A Two-sector Housing/population Model
  • A resort community in Colorado has determined
    that population growth in the area depends on the
    availability of hoousing as well as the
    persistent natural attractiveness of the area.
    Abundant housing attracts people at a greater
    rate than under normal conditions. The opposite
    is true when housing is tight. Area Residents
    also leave the community at a certain rate due
    primarily to the availability of housing.

69
Two-sector Population/housing Model, Continued
  • The housing construction iindustry, on the other
    hand, fluctuates depending on the land
    availability and housing desires. Abundant
    housing cuts back the construction of houses
    while the opposite is true when the housing
    situation is tight. Also, as land for
    residential development fills up (in this
    mountain valley), the construction rate decreases
    to the level of the demolition rate of houses.

70
What are the main sectors and how do these
interact?
  • Population
  • Housing

71
What is the structure within each sector?
  • Determine state/rate interactions first
  • Determine necessary supportng infrastructure
  • PARAMETERS
  • AUXILIARIES

72
What does the structure within the population
sector look like?
  • RATES in-migration, out-migration, net death
    rate
  • STATES population
  • PARAMETERS in-migration normal, out-migration
    normal, net death-rate normal

73
What does the structure within the housing sector
look like?
  • RATES construction rate, demolition rate
  • STATES housing
  • AUXILIARIES Land availability multiplier, land
    fraction occupied
  • PARAMETERS normal housing construction, average
    lifetime of housing
  • PARAMETERS land occupied by each unit, total
    residential land

74
What is the structure between sectors?
  • There are only AUXILIARIES, PARAMETERS, INPUTS
    and OUTPUTS

75
What are the between-sector auxiliaries?
  • Housing desired
  • Housing ratio
  • Housing construction multiplier
  • Attractiveness for in-migration multiplier
  • PARAMETER Housing units required per person

76
Can you construct the schematic model for this
Causal model?
77
We know what that is
78
How about this one?
79
We know what it is
80
Some rules
  • There are two types of causal links in causal
    models
  • Information
  • Flow
  • Information proceeds from stocks and parameters
    toward rates where it is used to control flows
  • Flow edges proceed from rates to states (stocks)
    in the causal diagram always

81
Loops
  • In any loop involving a pair of quantities/edges,
  • one quantity must be a rate
  • the other a state or stock,
  • one edge must be a flow edge
  • the other an information edge

82
CONSISTENCY
  • All of the edges directed toward a quantity are
    of the same type
  • All of the edges directed away from a quantity
    are of the same type

83
Rates and their edges
84
Parameters and their edges
85
Stocks and their edges
86
Auxiliaries and their edges
87
Outputs and their edges
88
STEP 1 Identify parameters/inputs
  • Parameters have no edges directed toward them

89
STEP 2 Identify the edges directed from
parameters
  • These are information edges always

90
STEP 3 By consistency identify as many other
edge types as you can
91
STEP 4 Look for loops involving a pair of
quantities only
  • Use the rules identified above

92
System Dynamics Software
  • STELLA and I think
  • High Performance Systems, Inc.
  • best fit for K-12 education
  • Vensim
  • Ventana systems, Inc.
  • Free from downloading off their web site
    www.vensim.com
  • Robust--including parametric data fitting and
    optimization
  • best fit for higher education
  • Powersim
  • What Arthur Andersen is using

93
What is system dynamics
  • A way to characterize systems as stocks and flows
    between stocks
  • Stocks are variables that accumulate the affects
    of other variables
  • Rates are variables the control the flows of
    material into andout of stocks
  • Auxiliaries are variables the modify information
    as it is passed from stocks to rates

94
A DEMO
95
Natures Templates the Archetypes
  • Structures of which we are unaware hold us
    prisoner
  • The swimmer scenario
  • Certain patterns of structure occur again and
    again called ARCHETYPES

96
We are creating a language
  • reinforcing feedback and balancing feedback are
    like the nouns and verbs
  • systems archetypes are the basic sentences
  • Behavior patterns appear again in all
    disciplines--biology, psychology, family therapy,
    economics, political science, ecology and
    management
  • Can result in the unification of knowledge across
    all fields

97
Recurring behavior patterns
  • Do we know how to recognize them?
  • Do we know how to describe them?
  • Do we know how to prescribe cures for them?
  • The ARCHETYPES describe these recurring behavior
    patterns

98
The ARCHETYPES
  • provide leverage points, intervention junctures
    at which substantial change can be brought about
  • put the systems perspective into practice
  • About a dozen systems ARCHETYPES have been
    identified
  • All ARCHETYPES are made up of the systems
    building blocks reinforcing processes,
    balancing processes, delays

99
Before attacking the ARCHETYPES we need to
understand simple structures
  • the reinforcing feedback loop
  • the balancing feedback loop
  • THE DEMO
  • Pages 520-525 in Austin/Burns--your handout

100
ARCHETYPE 1 LIMITS TO GROWTH
  • A reinforcing process is set in motion to produce
    a desired result. It creates a spiral of success
    but also creates inadvertent secondary effects
    (manifested in a alancing process) that
    eventually slow down the success.

101
Management Principle relative to ARCHETYPE 1
  • Dont push growth or success remove the factors
    limiting growth

102
ARCHETYPE 1 LIMITS TO GROWTH
  • Useful in all situations where growth bumps up
    against limits
  • Firms grow for a while, then plateau
  • Individuals get better for a while, then their
    personal growth slows.
  • Falling in love is kind of like this
  • The love begins to plateau as the couple get to
    know each other better

103
Structure
104
Understanding the Structure
  • High-tech orgs grow rapidly because of ability to
    introduce new products
  • This growth plateaus as lead times become too long

105
How to achieve Leverage
  • Most managers react to the slowing growth by
    puching harder on the reinforcing loop
  • Unfortunately, the more vigorously you push the
    familiar levels, the more strongly the balancing
    proces resists, and the more futile your efforts
    become.
  • Instead, concentrate on the balancing
    loop--changing the limiting factor
  • This is akin to Goldratts Theory of
    Constraints--remove the bottleneck, the impediment

106
Applications to Quality Circles and JIT
  • Quality circles work best when there is
    even-handed emphasis on both balancing and
    reinforcing loops
  • JIT has had to focus on recalcitrant suppliers
  • THERE WILL ALWAYS BE MORE LIMITING PROCESSES
  • When once source of limitatiin is removed,
    another will surface
  • Growth eventually WILL STOP

107
Create your own LIMITS TO GROWTH story
  • Identify a limits to growth pattern in your own
    experience
  • Diagram it
  • What is growing
  • What might be limitations
  • Example--the COBA and University capital
    campaigns
  • NOW, LOOK FOR LEVERAGE

108
Test your LIMITS TO GROWTH model
  • Talk to others about your perception
  • Test your ideas about leverage in small real-life
    experiments
  • Run and re-run the simulation model
  • Approach possible resistance and seek WIN-WIN
    strategies with them

109
ARCHETYPE 2 shifting the burden
  • An underlying problem generates symptoms that
    demand attention. But the underlying problem is
    difficult for people to address, either because
    it is obscure or costly to confront. So people
    shift the burden of their problem to other
    solutions--well-intentioned, easy fixes that seem
    extremely efficient. Unfortunately the easier
    solutions only ameliorate the symptoms they
    leave the underlying problem unaltered. The
    underlying problem grows worse and the system
    loses whatever abilities it had to solve the
    underlying problem.

110
The Stereotype Structure
Symptiom-Correcting Process
Addictioin Loop
Problem-Correcting Process
111
Special Case Eroding Goals
  • Full employment meant 4 unemployment in the 60,
    but 6 to 7 unemployment in the early 1980s
  • Gramm-Rudman bill called for reaching a balanced
    budget by 1991, but this was shifted to 1993 and
    from 1993 to 1996 and from 1996 to 1998
  • If all else fails, lower your goals..

112
EXAMPLE
113
Another Example
Raise tuition, add course fees, etc.
Costs of Higher Ed not funded by State
Perceived cost to the student
Lower enrollments
114
Still Another Example
Symptom-correcting process
Addiction Loop
Problem-correcting Process
115
Shifting the Burden is an insidious problem
  • Is has a subtle reinforcing cycle
  • This increases dependence on the symptomatic
    solution
  • But eventually, the system loses the ability to
    apply the fundamental solution
  • The system collapses

116
Senge Says
  • Todays problems are yesterdays solutions
  • We tend to look for solutions where they are
    easiest to find

117
HOW TO ACHIEVE LEVERAGE
  • Must strengthen the fundamental response
  • Requires a long-term orientation and a shared
    vision
  • Must weaken the symptomatic response
  • Requires a willingness to tell the truth about
    these solutions

118
Create your own Shifting the Burden Story
  • Is there a problem that is getting gradually
    worse over the long term?
  • Is the overall health of the system gradually
    worsening?
  • Is there a growing feeling of helplessness?
  • Have short-term fixes been applied?
  • The Casa Olay problem of using cupouns to
    generate business and then cant get away from
    using the coupons because their customer base is
    hucked on coupons

119
To structure your problem
  • Identify the problem
  • Next, identify a fundamental solution
  • Then, identify one or several symptomatic
    solutions
  • Finally, identify the possible negative side
    effects of the symptomatic solution

120
Review
  • We have now seen two of the basic systems
    archetypes.
  • The Limits to Growth Archetype
  • The Shifting the Burden Archetype
  • As the archetypes are mastered, they become
    combined into more elaborate systemic
    descriptions.
  • The basic sentences become parts of paragraphs
  • The simple stories become integrated into more
    involved stories

121
Seeing Structures, not just Trees
  • Helps us focus on what is important and what is
    not
  • Helps us determine what variables to focus on and
    which to play less attention to

122
WonderTech The Chapter 7 Scenario
  • A lesson in Growth and Underinvestment
  • What Senge gets out of this is the Growth and
    Underinvestment Archetype
  • A combination of variants of the Limits to Growth
    Archetype and the Shifting the Burden Archetype

123
The WonderTech Scenario
  • WonderTech continues to invest in the growth side
    of the process. Sales grow but then plateau.
    Management puts more sales people into the field.
    Offers more incentives to sales force. But
    because of long lead times, customers wane. Yes
    you have a great product, but you cant deliver
    on your lead time promise of eight weeks. We
    know weve heard from your other customers.
    In fact, the company relaxed its lead-time
    standard out to twelve to sixteen weeks because
    of insufficient capacity.

124
The Reinforcing Loop
125
The Balancing Loop Following the LTG Archetype
126
The Growth Curve Page 117
127
Whats happened?
  • WTs management did not pay much attention to
    their delivery service. They mainly tracked
    sales, profits, market share and return on
    investment. WTs managers waited until demand
    fell off before getting concerned about delivery
    times. But this is too late. The slow delivery
    time has already begun to correct itself. The
    management was not very concerned about the
    relaxed delivery time standard of eight weeks.

128
The WonderTech Scenario
  • The firm decides to build a new manufacturing
    facility. But the facility comes on line at a
    time when sales are declining and lead times are
    coming back to the eight-week standard.
  • Of every 10 startup companies, 5 will disappear
    with five years, only 4 survive into their tenth
    year and only 3 into their fifteenth year.

129
The Shifting the Burden Component
130
Put the whole thing together
131
Comments on The Senge Methodology
  • Sees problems as conforming to a finite number of
    archetypes
  • Formulates models based on combinations of the
    archetypes
  • Addresses problem-driven situations
  • What about situations and systems that are
    technology-driven, dynamics-driven,
    exogenously-driven, anything but problem-driven

132
More Comments on the Senge Methodology
  • But does this become sufficiently general to
    accommodate all dynamical scenarios and
    situations?
  • It is difficult to translate his archetypes and
    causal models into running system dynamics
    simulations
  • A lot of variables (RATE VARIABLES, specifically)
    get left out in terms of connections

133
More Comments on the Senge Methodology
  • The focus is on characterizing the dynamics, not
    on how to capture that in terms of stocks, flows
    and information paths
  • He doesnt label his edges with or - signs

134
Another methodology The Sector Approach to SD
model formulation
  • Begin by identifying the sectors
  • A sector is all the structure associated with a
    single flow
  • There could be several states in a single sector
  • Determine the within-sector structure
  • Reuse existing molecules where possible
  • Determine the between-sector information
    infrastructure
  • There are no flows and therefore no stocks or
    rates here

135
A Single-sector Exponential goal-seeking Model
  • Sonya Magnova is a television retailer who wishes
    to maintain a desired inventory of DI television
    sets so that she doesnt have to sell her
    demonstrator and show models. Sonyas ordering
    policy is quite simple--adjust actual inventory I
    toward desired inventory DI so as to force these
    to conform as closely as possible. The initial
    inventory is Io. The time required for ordered
    inventory to be received is AT.

136
A Two-sector Housing/population Model
  • A resort community in Colorado has determined
    that population growth in the area depends on the
    availability of hoousing as well as the
    persistent natural attractiveness of the area.
    Abundant housing attracts people at a greater
    rate than under normal conditions. The opposite
    is true when housing is tight. Area Residents
    also leave the community at a certain rate due
    primarily to the availability of housing.

137
Two-sector Population/housing Model, Continued
  • The housing construction iindustry, on the other
    hand, fluctuates depending on the land
    availability and housing desires. Abundant
    housing cuts back the construction of houses
    while the opposite is true when the housing
    situation is tight. Also, as land for
    residential development fills up (in this
    mountain valley), the construction rate decreases
    to the level of the demolition rate of houses.

138
What are the main sectors and how do these
interact?
  • Population
  • Housing

139
What is the structure within each sector?
  • Determine state/rate interactions first
  • Determine necessary supportng infrastructure
  • PARAMETERS
  • AUXILIARIES

140
What does the structure within the population
sector look like?
  • RATES in-migration, out-migration, net death
    rate
  • STATES population
  • PARAMETERS in-migration normal, out-migration
    normal, net death-rate normal

141
What does the structure within the housing sector
look like?
  • RATES construction rate, demolition rate
  • STATES housing
  • AUXILIARIES Land availability multiplier, land
    fraction occupied
  • PARAMETERS normal housing construction, average
    lifetime of housing
  • PARAMETERS land occupied by each unit, total
    residential land

142
What is the structure between sectors?
  • There are only AUXILIARIES, PARAMETERS, INPUTS
    and OUTPUTS

143
What are the between-sector auxiliaries?
  • Housing desired
  • Housing ratio
  • Housing construction multiplier
  • Attractiveness for in-migration multiplier
  • PARAMETER Housing units required per person

144
(No Transcript)
145
(No Transcript)
146
Experiments with growth models
  • Models with only one rate and one state
  • Average lifetime death rates
  • cohorts
  • Models in which the exiting rate is not a
    function of its adjacent state
  • Including effects from other variables
  • ratios and table functions
About PowerShow.com