1. Introduction to Computing
for Complex Systems
(Lab Session 9)

2. Goals For Today
Quick Overview of System Dynamics
Quick Overview of NOVA Programming
Concepts
Simple Population Growth Model

3. System Dynamics
Modeling

4. System Dynamics Modeling
Professor Forester also responsible
for inventing random-access
magnetic-core memory (RAM)
In 1950‘s Jay Forester of MIT
Business School applied ideas in
engineering to study business
dynamics

5. Feedback Effects
System Dynamics is interested in
systems that feature feedback effects
Feedback = the return to the input of a part of the output
(can be +, - or 0 )

6. negative feedback
negative feedback --> negative if the resulting action
opposes the condition that triggers it
This class of feedback is often described as auto-
regulating in so much as deviations from the equilbriua
are dragged back

7. System Dynamics
positive --> if the resulting action builds upon the
condition that triggers it
These are the more interesting class
Perturbations to the system can generate a novel set
of outcomes

8. An Example of Feedback
A positive connection:
“The positive connection for a cooling coffee cup
implies that the hotter the coffee is the faster it cools.
The variables Tc and Tr are coffee and room
temperature respectively.”
For Full Example:
http://serc.carleton.edu/introgeo/models/
loops.html

9. a negative connection:
“the negative connection in the figure below for a
cooling coffee cup implies a positive cooling rate
makes the coffee temperature drop.”
For Full Example:
http://serc.carleton.edu/introgeo/models/
loops.html
An Example of Feedback

10. An Example of Feedback
the two connections are
combined yield a
negative
feedback
loop
coffee temperature approaches the stable equilibrium of
the room temperature.
going around the loop the positive connection times the
negative connection gives a negative loop feedback
effect.

11. System Dynamics
System Dynamics Models are typically represented
by stocks and flows
http://www.systemdynamics.org

12. Basic Ideas,
Basic Ingredients

13. Basic ideas, Basic Ingredients
• Like agent based modeling, system
dynamics modeling is another method
used to study the behavior of complex
systems.

14. Basic ideas,
Basic Ingredients
• The dynamics of these systems are
generated by the interdependent nature of
their parts, especially concerning the role
of feedback loops between entities that
change over time, and the various
relationships that affect the rate of change
over time.

15. • The relationships described by system
dynamics models offer us a better picture
into how these systems often exhibit
nonlinear behavior and produce emergent
phenomena.
Basic ideas,
Basic Ingredients

16. Basic ideas, Basic Ingredients
• System dynamics models are
distinguished by the use of stocks, flows,
feedback loops.

17. Basic ideas, Basic Ingredients
• Stocks are things that either accumulate or
deplete over time, but which at any given
point in time can be measured.
• Flows are measured over a period of time;
they represent the rate of change in a
stock.

18. Example: Simple Population
Growth
• Let’s take a quick look at what a system
dynamics model would look like before we
build some for ourselves.
• Let’s model a population that grows, but
has no outlet for depletion.

19. Example: Simple Population
Growth
• First, we will need an accumulator, a
stock. Let’s call our stock “Population,”
since a population is something that can
be measured at a given time, but which
can also grow or decrease over time.

20. Stock: Population

21. Example: Simple Population
Growth
• Now we will want to manipulate our stock
over time. Let’s add a flow coming into our
stock so that we can put new things into it
over time. Since we are not going to
remove things from our stock, there is no
reason to place a flow leaving the stock.

22. Adding an In Flow

23. Example: Simple Population
Growth
• You may have noticed the hour glass
shaped object on the inflow. Stock and
flow diagrams often use this symbol to
represent a valve controlling the rate of
flow into or out of stocks.

24. Flows represent the rate of
change

25. Example: Simple Population
Growth
• Let’s call our flow “Growth.” This will
represent the amount of new population
members flowing in to the population
during each time step.

26. Growth Added

27. Example: Simple Population
Growth
• To finalize our model, we will want to inform
the growth flow of the prior population size as
well as the current rate of growth.
• After adding those last elements we will have
finished our qualitative description of a simple
population growth model!

28. Simple Population Growth

29. • System dynamics modeling programs
allow us to mathematical define the
relationships between our stocks and
flows, allowing us to simulate model runs
and see what occurs in the system when
we tweak parameter and variable values.
Basic ideas, Basic Ingredients

30. • The program we will be using to create
system dynamics models is called NOVA.
Introduction to NOVA

31. http://www.novamodeler.com/

32. http://www.novamodeler.com/

33. http://www.novamodeler.com/

35. Introduction to NOVA
• “Nova is a dynamic modeling platform that
supports and encourages hierarchical
design. Through hierarchical design, Nova
combines the dynamic systems (DS),
spacial and agent-based modeling
paradigms.”
http://www.novamodeler.com/

36. Introduction to NOVA
• NOVA is being developed by Professor
Richard Salter of Oberlin College
• NOVA takes an interesting approach to
system dynamics modeling by enabling
modelers to easily create sub-procedures
that are also system dynamics models.

37. Introduction to NOVA
• This features gives modelers flexibility and
scalability in modeling dynamic
relationships between different interacting
systems as well as the ability to quickly
duplicate dynamic systems of interest,
either in a higher level of the same model,
or in other models.

38. Introduction to NOVA
• Like writing good modular code in Netlogo
or other programming languages, the sub-
models of NOVA provide an excellent way
to organize models and allow users to
utilize relevant sub-procedures of other
models within their own programs.

39. Quick Overview of Nova
Concepts
• Helpful Sites:
• The following website features
explanations of NOVA modeling
components, as well as tutorials.
• https://sites.google.com/a/oberlin.edu/
nova-help/

40. Quick Overview of Nova
Concepts
• Helpful Sites:
• The following website features several
NOVA models that can run through Java
Web Start.
• https://sites.google.com/a/oberlin.edu/
nova/00-nova-concepts

41. Quick Overview of Nova
Concepts
• Let’s go through a quick run down of some
of NOVA’s components before we build a
model of our own.
• First, open the Nova.exe application file in
your Nova_Windows folder.

42. Open Nova Application

43. Quick Overview of Nova
Concepts
• You should now have an open window;
the large empty area on the left is known
as the canvas, which is where models are
built
• As we will see later, the boxes to the left
are where the main model and sub-models
are located

45. Quick Overview of Nova
Concepts
• Let’s look at where we will locate the many
of the things needed to build system
dynamics models.
• There are three buttons on the top right
that we should look at: Components,
Plugins, and the Simulator

46. First Click on Components

47. Components
• The components contain the building
blocks of the models, such as the stocks
and flows
• It also contains items unique to NOVA,
such as the cell grid, which creates a
homogeneous multidimensional array of
submodels that can interact with each
other.

48. Components Menu

49. Plugins
• The next button to look at is the Plugins.
• This contains the different types of GUI’s that
we will use to visualize our models with.
• Plugins can also be developed and
specialized for a user’s model and added to
NOVA’s Plugin directory.

50. Plugins
• Plugins can also be developed and
specialized for a user’s model, as well as
added to NOVA’s Plugin directory.
• Not all Plugins deal with visualizations, for
example, one creates histograms.

51. Plugins Menu

52. Simulator
• Finally, let’s investigate the Simulator
• The simulator offers several different
means of operating model runs.

53. Clock
• The first way to run a model is to use
clock.
• It allows users to start a model run, pause
it, stop it, reset it, speed it up or slow it
down, and also turn looping on and off.

54. Auto
• The second way to run a simulation is
through Auto mode, which reruns the
simulation any time there is a change of input
using a slider or spinner.
• This is highly useful when you want to
visually see what changing a given
parameter can do.

55. Repeater
• Finally, the Repeater is used primarily for
sensitivity analysis, to create multiple runs
with varying parameter values.

56. The Simulator

57. Model Library
• You can find other NOVA models in the
NOVA model library, which is under File >
Browse Model Library

59. NOVA Help
• If you forget what a component or plug-in
does, just left click it and then press ‘F1.’
• This will bring up the NOVA help website
which will explain what that component
does.

60. Applying Model Building
Principles Across
Different Platforms

61. This ought to look familiar…
• Although system dynamics provide a
different way of approaching complex
systems, many of the lessons you have
learned so far about programming agent
based models in Netlogo should readily
apply, especially in the NOVA
environment.

62. This ought to look familiar…
• For example, let’s look at the flocking
model, found in the NOVA Model Library
under the Agent-based model section.
• Since it is a new environment, it is not
surprising that the model may at first look
strange and intimidating…

64. Applying Model Building Principles
Across Different Platforms
• However, I think if you take a closer look,
you’ll start to see some similarities…

65. NOVA

66. Netlogo

67. NOVA

68. Net
Logo

69. Applying Model Building Principles
Across Different Platforms
• You’ve mapped the dependencies in the
code of the flocking model of Netlogo
before, so I think you can see that while
there are differences, some basic
principles can be applied across different
modeling platforms!

70. Similarities Across Programs
• In Netlogo, there are sub-procedures to
handle a bird’s aligning, cohering, or
separating,
• As you saw, there are similar
dependencies in the NOVA system
dynamics model version.

71. Similarities Across Programs
• Although the flocking model is an
advanced model, your prior experience
with it in Netlogo should make what once
looked strange and unfamiliar seem much
more manageable.

72. Modular Coding and
Hierarchical Systems
• Another thing to note is that NOVA’s
hierarchical design works just like modular
coding in Netlogo.
• Even better, as we will see when we
construct our own NOVA model, NOVA
allows you to literally drag and drop sub-
models into other models.

73. Modular Coding and
Hierarchical Systems
• The drag-and-drop feature is just like
adapting already existing code in Netlogo to
your own modeling needs.
• If you needed the “Mover” sub-model in the
NOVA flocking model, which converts polar
coordinates to rectilinear coordinates, you
can copy it to your own model.

74. Modular Coding and
Hierarchical Systems
• You do not need to reinvent the wheel.

75. Building Our Own Model
• Now we are ready to build our own system
dynamics model in NOVA.
• Since we have already conceptually mapped
what a simple population growth model would
look like, why don’t we expand upon that and
see what implementing a system dynamics
model looks like!

76. Building the Simple
Population Growth
Model

77. Building Our Own Model
• If you haven’t done so already, let’s get a
clear NOVA canvas to work with.
• Go to File > New Project to bring up a
blank page to work with.

78. File > New Project

80. Building the Model
• As you remember from before when we
conceptually built the simple population
growth model, we know that we will need a
stock for population, a flow for growth, and
parameters that will tell the model what the
growth rate is at any given time.

81. Building the Model
• Let’s get components for our model.
• Please click on the Components button on
the top right of the screen to access the
Components Menu.

82. Building the Model
• We are going to want to get a stock.
• Stocks are represented by the green
square in the first top left button in the
Components menu.

83. Building the Model
• Now that you have pressed stock, click on
any clear section of the NOVA canvas.
• The program will ask you to name the stock.
Please name it “Population”

84. Building the Model
• If you do not give a Stock a name, NOVA
will assign it the name “Stock 0.” This is
true for other component types as well.
• If you need to rename a component, just
double click on the name (not the
component, just the name tag).

85. Building the Model
• You should now have a stock called
“Population.”

86. Building the Model
• The next thing we need is a flow. Click on the
flow button, which is the button on the very
top far right that looks like a yellow hour
glass.
• As we mentioned before, it represents a
valve controlling the rate of the flow.

87. Building the Model
• Now click somewhere to the left of the
population stock.
• Next, NOVA will ask you to assign the flow
a name. Give the flow the name, Growth.

88. Building the Model
• You should now have something that
looks like this:

89. The Select, Resize, & Move
Arrow
• Before we move on, it would be a good thing
to learn about an arrow you can use to select
and move components with. This arrow is on
the top right, next to the blue question mark
and green ‘i’ button.

90. The Select, Resize, & Move
Arrow
• By forming a rectangle completely around
any component you wish to move, this arrow
will allow you to move any object or objects
you wish.

91. The Select, Resize, & Move
Arrow
• The objects will become black when they
are selected. While they are this color, you
may move them.

92. Another Way to Move Objects
• Even without the move arrow, you can
simply click on a component once to move
it. Like with the move arrow, after a single
click, the object will turn black.

93. The Select, Resize, & Move
Arrow
• Sometimes, when you are selecting
components, you might forget that you still
have a component button turned on, and this
will cause you to accidently put down
components.
• The Select & Move arrow is a good ‘neutral’
position to be in to stop that from occurring.

94. • Back to our model…

95. Building the Model
• Again, you should now have something
that looks like this:

96. Building the Model
• The next step is to connect the Growth Flow
rate to our population stock.
• However, first we should look inside the flow
rate, to see how flow rates work, and to make
the flow rate uniflow (since it will only be
moving in one direction, into the stock.)

97. Building the Model
• Right-click on the valve of the Growth Flow
rate, you should get the following
interface:

99. Building the Model
• This is where you will be able to define the
mathematical relationships between stocks
and flows.
• You can see that on the bottom left, there is a
list of programming code that is native to
NOVA, just like the ‘primatives’ of Netlogo.

101. Building the Model
• The box on the top right will be where other
variables and parameters that affect the rate
of the flow will show up.
• This will you will know what needs to be
accounted for when you write out the
equation detailing how the flow will influence
your stock.

103. Building the Model
• The large box in the top middle is where the
code will be written.
• Just to give a hint of what is to come, in order
to model population growth we will have to
come up with an equation using the following
terms: current population and the current
growth rate.

105. Building the Model
• Finally, the bottom middle box is for
comments.
• Just like commenting in Netlogo was not
used by the program, anything written
here is for users to see only, and will not
alter the operation of the model in any
way.

107. Building the Model
• Under the comments box you will see two
options, Uniflow and Biflow.
• Select Uniflow and then press OK.

108. Building the Model
• Now we will connect our Growth Flow to
our Population Stock.
• First double-click on the right-hand cloud.
You should see a blue dotted line that
follows your mouse wherever it goes.

110. Building the Model
• The blue dotted line is ‘sticky,’ it will not let
go of your mouse until it is stuck onto a
stock. Right now, that is exactly what we
want. However, sometimes, especially if
you double click a cloud by mistake, this
can be a nuisance.

111. Building the Model
• As mentioned earlier, selecting the Select
and Move arrow is a good way to get out of
such jams.
• Another way is to click the bottom right edge
of the components or plugins box. This will
get rid of the sticky blue dotted line, as well
as other component buttons.

113. Building the Model
• Now we will connect the flow to the stock.
• Double-click the right-hand cloud again and
drag the blue dotted line to the population
box. Once there click the population stock.

115. Building the Model
• Your model show look like the following:

117. Building the Model
• Now our flow is connected to our stock!
• We now have a stock, and something to
change the stock.

118. Building the Model
• Since we are building an exponential
growth model, we need to inform the flow
of the existence of the current population,
since the current growth rate is dependent
upon the current population size.

119. Building the Model
• This is done by ‘adding arrows,’ which
connect an information source to another
component that needs the information.

120. Building the Model
• Go back to the components menu and
select the red arrow button
• It is on the left hand column of the 2nd to
the top row.

121. Sticky Lines!
• WARNING!
• Red arrows are sticky too. If you need to
get out of red arrow mode, just select the
select and move arrow, or click the bottom
right edge of the components menu.

122. Building the Model
• Once you clicked the red arrow, click on the
Population Stock. There should now be a red
dotted line following your mouse coming out
of the stock.

123. Building the Model
• Drag your mouse, and the red dotted line, to
the yellow valve on the Growth Flow and click
it. There will not be a red arrow pointing from
the Population Stock to the flow.

124. How do I get Rid of
Red Arrows?
• If you have a red arrow you don’t like, just
click off the red arrow (by clicking the bottom
right edge of the components menu) and
then click the circle on the end of the red
arrow.
• It will turn the arrow black, and you can
either move the arrow or delete it.

125. Building the Model

126. Building the Model
• Now we need a rate of growth.
• We will allow the user to select what the
rate of growth will be, so we want to use a
component that allows for user input.

127. Building the Model
• Go back to the components menu and
select the button in the middle column of
the second to the top row. This button
looks like a red arrow pointing away from
the left.

128. Building the Model
• This is a data input button.
• We are actually creating the sub-model of
our main model right now. The rate of
growth slider will be in the main model’s
interface, and this input button brings in
the information into the sub-model.

129. Building the Model
• Call the Data Input button ‘rate.’
• Now we want to connect the rate to the
Growth Flow.
• Go back to the components menu and get
a red arrow and connect it to ‘rate.’

130. It Should Look Like This

131. Building the Model
• Now connect the red arrow to the yellow
growth valve. Now we have information about
the current population size and the rate of
growth going into the flow.

132. NOW Time to
ADD THE MATH …
J

133. It’s Getting Serious Now
• Right-click once on the yellow valve on the
Growth Flow.
• This will take you back to the Flow
Interface.
• We are going to add an equation in the
large box in the middle.

135. Building the Model
• Notice how the stock and data input are
now in the box on the top left.
• That is because the red arrows are
pointing to the flow.

136. Building the Model
• You can now either write “rate * population”
• OR
• You can click rate in the top left box, type *,
and then click population. Clicking them
ensures you won’t misspell anything.

137. It Will Look Like This:

138. Building the Model
• If you misspell a word, the flow’s name
becomes red.

139. Building the Model
• We would also like to add an initial
population, especially one that the user
could select.
• In order to do that we return back to the
components menu, and select the Data
Input button again.

140. Building the Model
• This time, click on a clear spot on the
canvas and name the Data Input, “Initial
Population”
• Go back to the components menu and
select the red arrow, click it on the Data
Input, “Initial Population,” and then
connect that arrow to the Population
Stock.

141. Here’s What It Should Look Like

142. Building the Model
• Once you have clicked out of red arrow
mode, go back and left click the Population
Stock.
• This will bring up the Stock Interface. Click on
Initial Population in the top left hand box, so
that Initial Population then appears in the top
middle box.

143. Building the Model
• Click on Initial Population in the top left
hand box, so that Initial Population then
appears in the top middle box. Then click
OK to get out of the interface.

144. Building the Model
• The next step is to bring the current
population information into the main model
interface.
• Just as Data Input buttons are used to bring
information from the main model to the sub-
model, we will bring the information to the
main model from the sub-model with a Data
Output button.

145. Building the Model
• Go back to the components menu, and
select the button in the right hand column
of the 2nd to the top row.

146. Building the Model
• Click somewhere to the right of the
population stock and name the Data
Output, “pop”.
• Then get a red arrow, connect it to the
Population Stock, and connect the arrow
to the Data Output, “pop.”

147. Our Sub-Model Is Almost Done

148. Building the Model
• There is only one more step before our
sub-modal is finished.
• Right-Click on the data output “pop”, and
write “Population” in the Data Output
Interface Coding box (the one in the
middle on the top). This sets “pop” to
“Population,” our stock.

150. Building the Model
• That’s it! Now the stock’s value will be
outputted to the main level of the model.
• Now we will create the main level so that
we can create the sliders and the GUI.

151. Finished Sub-Model

152. Building the Main Model
• To create the main level of the model, we
need to click the yellow paper button on
the top left.

153. Building the Main Model
• It will ask you to name the sub-model. Call
it “Simple Population Growth”

154. Building the Main Model
• Now there is probably a blank screen. This
is the higher level of the model. It has not
yet been made.

155. Building the Main Model
• You’ll see the sub-model in the bottom left
box. Click on the name and you’ll get our
sub-model.

156. Building the Main Model
• We can give our main model a title. Just
right click on “Untitled” and give it the
name “Population Model”.

158. Building the Main Model
• Now click on “Population Model,” the main
model.
• We are about the drag-and-drop our sub-
model into the main model.

159. • Click, AND HOLD, the sub-model’s name
in the bottom left box (Simple Population
Growth), and drag your mouse, while
holding down, until you get into the empty
canvas. Now drop.
• You now have the sub-model embedded
as a chip inside the main model.
Building the Main Model

160. Drag-and-Dropping the Sub-
model

161. Seeing the Sub-Model from the
Main Model Level
• To see the sub-model, just press Shift + right click
and press on the chip.

162. Building the Main Model
• Did you see the little red things sticking out
of the chip? Those are the two data inputs
and the one data output!
• They are called pins, and they
communicate with the sub-model.

163. Building the Main Model
• Which means we need sliders for the data
inputs.
• Go to components and select the slider (its
on the left hand side, 4th row down).

164. Building the Main Model
• Once you selected slider, click somewhere
clear on the canvas and name the slider,
“rate”.

166. Building the Main Model
• The slider goes between 0 and 100.
• Those are bad values for a growth rate.
• Right click the bottom right edge to get the
slider interface, and change the high value to
2.0.

168. Building the Main Model
• Create another slider and call it “Initial
Population.”
• You can leave the range between 0 and
100.

169. Building the Main Model
• However, we cannot have fractions of
people in the initial population.
• Right click on the bottom right edge of the
slider to change the decimal place to 0.

171. Building the Main Model
• We can now assign the sliders to the input
pins in out sub-model chip in the main
model.
• Go ahead and right click on the sub-model
chip and an interface will appear.

173. Building the Main Model
• Go ahead and assign the input “rate” to
“rate” in the draw down menu, and the
input “Initial_Population” to Initial
Population in the draw down menu.
• These connect the inputs in the sub-model
to the main model’s sliders. No red sticky
arrows needed.

174. Our Main Model, so far

175. Our Model, so far
• Our sub-model calculates the new
population is each time step, and with the
new sliders, we are able to input an initial
population, and a current population
growth rate.
• All we need now is a graph!

176. Getting the Graph
• Go to the components menu and select
the ‘add graph’ button, located on the left-
hand side on the 2nd to the bottom row.

177. Getting the Graph
• Now add the graph somewhere to the right
of the sub-model chip.
• Name the graph, “Population.”

178. The Graph, Population

179. Getting the Graph
• We need to configure the graph.
• Right click on the right bottom edge of the
graph to pull up the graph interface.

181. Getting the Graph
• As you can see, the interface wants us to now
select what we want to graph.
• In the left-hand “Select Contents,” there is an
option labeled, Simple Population
Growth_0.pop
• Clearly, that is what we want to select, but
why?

182. Getting the Graph
• Simple Population Growth is the name of
the sub-model.
• The “.pop” refers to the output pin, which
you will remember we named, “pop.”

183. Sub-Models Can Be Used by
Multiple Chips, Multiple Models
• It is assigned the number zero because that is
the name of the chip in our interface. We could
create another chip, and then we would have
two models, each using the same sub-model,
but we could supply different sliders in order to
provide different values.

184. Getting the Graph
• We can give an alias what we are
graphing by selecting Simple Population
Growth_0.pop, and then writing an alias in
the Alias box, and pressing Set.
• Name the alias, “Population Model”

185. Getting the Graph

186. Our Model Output!
A Success!

187. The Finished Product
• Our model now is fully linked up to the
graph, and therefore has a complete two-
tiered hierarchical system dynamics model
of exponential growth!

188. The Finished Product

189. Running the Simulation
• Now go to the Simulator and select Clock
• As we said earlier, Clock runs for a period
of time given certain parameters.
• Go ahead and run the model with different
parameters

190. Running the Simulation
• You can also select Auto in the Simulator.
• Auto will automatically update when you
change parameters
• Go ahead an run a few different
parameters using Auto

191. A Word of Advice
• While not so important with graphs, be
careful about fussing too much with the
GUIs while they run.
• It can cause problems.