Vensim chapter 6 v1

Vensim
e s
Chapter 6
Building a Simulation Model

Vensim Chapter 6: Building a Simulation Model Presented by Diana Lim December 2010

Naming Conventions
g
Levels – initial capital letters; e.g., Population
Rates, auxiliaries, constants, lookups, data variables – all lower
Rates auxiliaries constants lookups data variables – all lower
case; e.g., average lifetime


Sketch Conventions
Sketch Conventions
Levels or stocks are entered with the Box Variable tool
Levels or stocks are entered with the Box Variable tool
Rates are entered with the Rate tool

Box Variable - Level Rate


Sketching the Rabbit Model
g
File > New Model

1 5
2
Type the name
Population and press
3 enter
t

4


Sketching the Rabbit Model
g

9

7 8 6

6 5 6

8

7 7

A positive feedback loop from Population to births increases
Population, and a negative feedback loop from deaths
decreases Population
decreases Population

Writing Equations
g q
births = population * birth rate
Birth rate has not causes; it is constant in the model
Bi th t h t it i t t i th d l

Equations All the variables in the
model will turn black.

The highlights indicate
which variables still
require equations or
have incomplete
equations


Checking y
g your model
Model > Units Check


Variable Type: Auxiliary
yp y
Click on variable births

2

1

3

4


Variable Type: Level
yp
Click on variable population
equation is already present in the equation editing box

Rates constructed by clicking first outside, and then on the Level
are considered positive (inwards) flows; rates constructed by
clicking on the Level, and then outside the Level are considered
2 negative (draining) flows

1

3

4

Variable Type: Constant
yp
birth rate

2

1

3 this means that the fractional birth
rate is measured in fraction (of
rabbits) per year.

4


Variable Type: Constant
yp
average lifetime

1

2


Variable Type: Auxiliary
yp y
Click on variable deaths

1

2

3


Checking y
g your model
Model > Check Model


Unit Equivalents ( y
q (Synonyms)
y )
Fail in Units check because Vensim does not see the plural
form as the same word
form as the same word
Model > Settings...

1

2

3


Simulating the Model
g
2
1
Double click on the Runname editing
box on the Toolbar and type equilib for
yp q
the first run name.


Model Analysis
y
Click on the Level Population in the sketch.
Click on the Graph t l A
Cli k th G h tool. A graph of Population i
h fP l ti is generated:
t d

2 1
Graph


Model Analysis
y

Population appears as a flat line at the top of the graph at
P l i fl li h f h h
1000 rabbits. To check that no change is occurring,
Click on the Table tool
l k h bl l
Output window
shows that
Population is
unchanging.


Simulation Experiments
p
Click on the SyntheSim button . Answer yes to
whether you want to overwrite the existing run.
whether you want to overwrite the existing run
The toolbar will change to

and you will see variable behavior for all elements on the
diagram.
di


Simulation Experiments
p
Click on the Runname editing box and replace equilib with the
name growth.
name growth
Drag the slider beneath birth rate up till it shows 0.2 as its
value.

The blue lines show the current run and the red line the
equilib run results
run results.
Click on the Stop button to stop SyntheSim.
View>Show Behavior to see behavior graphs again.
Vi >Sh B h i t b h i h i

Alternative Simulation Setup
p
Click on the Set up a Simulation button
The Toolbar changes to the simulation toolbar.
The Toolbar changes to the simulation toolbar

notice that the constants turn into yellow words with blue
ti th t th t t t i t ll d ith bl
background
Also, the sketch tools are grayed out, preventing work in the
Also the sketch tools are grayed out preventing work in the
sketch window.
Click on the variable birth rate
and in the editing box
type the value 0.2

This is a temporary change for
this run only and does not
permanently alter the value in
your model.
d l


Causes Strip Graph
p p
Click on Population to select it into the workbench.
Click on the Control Panel button
Cli k th C t l P l b tt
1

2

The last run you made (growth)
is loaded first (at the top of the
dialog).


Causes Strip Graph
p p
Click on the Graph tool. A graph will show both runs


Causes Strip Graph
p p
Now click on the Causes Strip tool.

Causes Strip


Runs Compare
p
Click on the Runs Compare tool.
shows the differences in the Constant birth rate for runs
equilib and growth.

Runs Compare

Population grew in the growth run because the birth rate was
set to a higher value than the equilibrium value. This made the
positive feedback loop through births stronger than the
positive feedback loop through births stronger than the
negative feedback loop through deaths, resulting in Population
growth over time.


Exponential Decay
p y
make changes to a model Constant to generate exponential
decay or decline in the population
decay or decline in the population
1
1
4 2

Drag the Slider under
average lifetime till it
displays
di l a value of 4
l f 4.

Click on the Stop
Button
3

Click the Simulate button and the model will simulate


Exponential Decay
p y
Click on the Graph tool and compare the three runs.
1

double click
on the run equilib in
the right box; this will
h i h b hi ill
unload the run so the
2 Analysis tools will
not examine it
it.


Exponential Decay
p y
Click on the Causes Strip tool
to show Population and its
to show Population and its
causes— births and deaths, for
the two last runs.
the two last runs

Population d li
P l i declines i the d
in h decay run
because the average lifetime was set to a
lower value than the equilibrium value.
This made the negative feedback loop
through deaths stronger than the positive
feedback loop through births, resulting in
p g g
Population decline over time.


Input and Output Objects
p p j
Click on the Control Panel button and then select the
Graphs tab
tab.
1

2

3


p p j

1

Do the same for
births (select births
from the list) and
f h li ) d
deaths (select deaths
3 from the list)
4
2
5

6 5


p p j
Select the Input Output Object sketch tool

4 2

1

3


p p j
Click the SyntheSim button
Experiment with moving the sliders around and seeing
E i t ith i th lid d d i
the results on the graph.


Vensim chapter 6 v1

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