University of Toronto
Department of Computer Science
CSC2720: Systems Thinking
for Global Problems
➜ Last
week:
Ä Why systems thinking?
Ä What sort of discipline is it?
Ä Defining a system
➜ This
week:
Ä Feedback Loops
Ä The idea of limits
Ä Multiple feedback loops
© 2012 Steve Easterbrook. This presentation is available free for non-commercial use with attribution under a creative commons license.
1
University of Toronto
Department of Computer Science
Elements of a System
© 2012 Steve Easterbrook. This presentation is available free for non-commercial use with attribution under a creative commons license.
2
University of Toronto
Department of Computer Science
General Systems Theory
➜  How
scientists understand the world:
Ä Reductionism - break a phenomena down into its constituent parts
Ø  E.g. reduce to a set of equations governing interactions
Ä Statistics - measure average behaviour of a very large number of
instances
Ø  E.g. gas pressure results from averaging random movements of zillions of atoms
Ø  Error tends to zero when the number of instances gets this large
➜  But
sometimes neither of these work:
Ä Systems that are too interconnected to be broken into parts
Ä Behaviour that is not random enough for statistical analysis
➜  General
systems theory
Ä Originally developed for biological systems:
Ø  E.g. to understand the human body, and the phenomena of ‘life’
Ä Basic ideas:
Ø  Treat inter-related phenomena as a system
Ø  Study the relationships between the pieces and the system as a whole
Ø  Don’t worry if we don’t fully understand each piece
© 2012 Steve Easterbrook. This presentation is available free for non-commercial use with attribution under a creative commons license.
3
University of Toronto
Department of Computer Science
Two approaches to study systems
1. 
Focus on the rules followed by individuals
(bottom up)
Ä 
Ä 
2. 
Study emergent behaviour
Build simulations (agent models, physics models, etc)
Focus on the patterns of behaviour of the
system as a whole (top down)
Ä 
Ä 
Build causal diagrams
Study feedback loops (system dynamics models)
© 2012 Steve Easterbrook. This presentation is available free for non-commercial use with attribution under a creative commons license.
4
Department of Computer Science
University of Toronto
Behaviour of Complex Systems
Many relationships
Simple rules
Complex
Non-linear
Systems
Large numbers
of interactions
Self-organizing
behaviour
© 2012 Steve Easterbrook. This presentation is available free for non-commercial use with attribution under a creative commons license.
5
University of Toronto
Department of Computer Science
Stocks and Flows Diagrams
© 2012 Steve Easterbrook. This presentation is available free for non-commercial use with attribution under a creative commons license.
6
Department of Computer Science
University of Toronto
Stocks and Flows Diagrams
R
B
© 2012 Steve Easterbrook. This presentation is available free for non-commercial use with attribution under a creative commons license.
7
University of Toronto
Department of Computer Science
Example Stock’n’Flow Diagrams
© 2012 Steve Easterbrook. This presentation is available free for non-commercial use with attribution under a creative commons license.
8
University of Toronto
Department of Computer Science
Balancing Feedback Loops
© 2012 Steve Easterbrook. This presentation is available free for non-commercial use with attribution under a creative commons license.
9
University of Toronto
Department of Computer Science
Reinforcing Loops
© 2012 Steve Easterbrook. This presentation is available free for non-commercial use with attribution under a creative commons license.
10
University of Toronto
Department of Computer Science
House Thermostat
© 2012 Steve Easterbrook. This presentation is available free for non-commercial use with attribution under a creative commons license.
11
University of Toronto
Department of Computer Science
But the room doesn’t stay warm!
© 2012 Steve Easterbrook. This presentation is available free for non-commercial use with attribution under a creative commons license.
12
University of Toronto
Department of Computer Science
Multi-loop systems
© 2012 Steve Easterbrook. This presentation is available free for non-commercial use with attribution under a creative commons license.
13
University of Toronto
Department of Computer Science
Rational markets?
© 2012 Steve Easterbrook. This presentation is available free for non-commercial use with attribution under a creative commons license.
14
University of Toronto
Department of Computer Science
© 2012 Steve Easterbrook. This presentation is available free for non-commercial use with attribution under a creative commons license.
15
Department of Computer Science
University of Toronto
Types of System
➜ 
Natural Systems
➜ 
Ä  E.g. ecosystems, weather, water cycle,
the human body, bee colony,…
Ä  Usually perceived as hard systems
➜ 
➜ 
Ä  E.g. families, businesses,
organizations, markets, clubs, …
Ä  E.g. any designed system when we also
include its context of use
Abstract Systems
Ä  E.g. set of mathematical equations,
computer programs,…
Ä  Interesting property: system and its
description are the same thing
Ø  Similarly for abstract and symbol
systems!
➜ 
Ø  Part of the design includes the
representation of the current state of
some human activity system
Symbol Systems
Designed Systems
Ä  E.g. cars, planes, buildings, freeways,
telephones, the internet,…
Information Systems
Ä  Special case of designed systems
Ä  E.g. languages, sets of icons,
streetsigns,…
Ä  Soft because meanings change
➜ 
Human Activity Systems
Ä  E.g. MIS, banking systems, databases,
…
➜ 
Control systems
Ä  Special case of designed systems
Ø  Designed to control some other system
(usually another designed system)
Ä  E.g. thermostats, autopilots, …
© 2012 Steve Easterbrook. This presentation is available free for non-commercial use with attribution under a creative commons license.
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