l4-sd stock flow

7/27/2019 L4-SD Stock Flow

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Stocks & Flows

Diagrams: BASICBased on : Budi HARTONO

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Agenda

Intro of SFD

Case 1 the Savings

Case 2 the Flu

Case 3 population dynamics

General Modelling Approach

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Introduction

Box Variables represent quantities. These are

the main "nouns" in a system and are

sometimes refered to as Level Variables.

Rates represent changes over time. These are

the main "verbs" in a system and are

sometimes refered to as Flows.

Auxiliary Variables represent constants and

other parameters. These loosely corespond to

"adjectives" and "adverbs" in a system.

Connectors indicate dependencies between

objects. In other words, I need to know this to

calculate that

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Steps

1. Building the Model

2. Specifying the Input

3. Running the Model

4. Examining the Output

5. Analysis & Insights

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Case 1 Interest Rate

Source: Vensim PLE Quick Reference and Tutorial, b: Craig W. Kirkwood,2002

CLD?STOCK?

FLOW?

EXOGENOUS

ENDOGENEOUS?

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1. Setting

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Units & syntax checks

Run Simulations

Graphs

Customized Graphs

Synthesim

Working with lookup functions

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Tips: Using Time as a variable

Current

Variable Demand

100

50

0

-50

-100Time

100

75

50

25

00 50 100

Time (Month)

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Case 2 The Flu

On a Monday, 100 students return toschool, one of whom has the flu. As time

passes, the sick student unwittinglyinfects his classmates, who in turn go onto infect others. This could be generalizedas follows:

Over time, susceptible people get sickand become infected people

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Building the Model - Two main _____sugeng purwoko


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Building the Model - Adding a__________

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Building the Model - Adding a______

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Building the Model - AddingConnectors

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Specifying the Input - DATAWhat data?sugeng purwoko

pec y ng e npu ep o


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pec y ng e npu - ep oxVariables Recall that our hypothetical school has 100 kids.

Set the initial number of susceptible people to 99and set the number of infected people to 1.

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Specifying the Input - Step 2 RateEquation

Assume that every susceptible personinteracts with every infected person once

per day. Not every interaction results in an

infection. Infection fraction.

Susceptible People * Infected People *Infection Fraction

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Specifying the Input - Step 3 TheInfection Fraction

Assuming that in two days there will bethree new sick students, the infection

fraction can be estimated as 0.0151

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Running the Model

But, before that ...

a. Model Settings

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St 1 Q i k G h d


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Step 1 Quick Graphs andTables

Causes Strip - This brings up a strip graph for the

selected variable and any variables affecting it in

a causal loop.

Graph - This brings up a graph of the selectedvariable over time.

Table - Displays values for the selected variable

horizontally.

Table Time Down - Displays values for the

selected variable over time vertically

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Examining the OutputStep 2 Custom Graphs and Tables

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Examining the OutputStep 3 SyntheSim

This button activates SyntheSim mode (A model must

be ready to run before you can enter SyntheSim).

This button exits SyntheSim mode.

This button resets the selected slider to the value it had

when SyntheSim was started.

This button resets all of the sliders in the model.

This button saves the current model settings and run

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Analysis

Graphs?

Peaks of diseases?

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Case 3 Population Dynamics

average lifetime = 8

Units: Year

birth rate = 0.125

Units: fraction/Year

births = Population * birth rate

Units: rabbit/Year

deaths = Population / average lifetime

Units: rabbit/Year

Population = INTEG(

births deaths, 1000)

Units: rabbit

Model Equations

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Running Simulation

Double click on the Runname editingbox on the Toolbar and type equilib

for the first run name. Click on the Simulate button

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Model Analysis

Causes Strip

Graph

Table

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Comparing Scenarios

Equilibrium vs. Exponential growth

1. Set Up a Simulation

2. .

3. Replace equilib with growth

4. Click on the variable birth rate (appearingblue/yellow in the sketch) and in the editing boxtype the value 0.2

5. Simulate

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Analysis

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Runs Compare

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 negative

feedback loop through deaths, resulting in Population growth over time

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Comparing Scenarios

Exponential Decay

decay to replace the growth

average lifetime = 4

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General Modelling Approach

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The Elements

General Modelling

Approach

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stocks, flows, and informationa general way of graphically representing

anybusiness process

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CHANGE !!

A primary interest of business managers ischanges in variables like Actual Customers overtime.

If nothing changes, then anybody can manage.just do what has always been done.

Some of the greatest management challengescome from change.

If sales start to decline, or even increase, you

should investigate why this change has occurredand how to address it. One of the key differences between managers

who are successful and those who are not istheir ability to address changes before it istoo late.

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Types of Stocks and Flows Most business activities include one or more of

the following five types of stocks

1. Materials. This includes all stocks and flows of physical goods which are

part of a production and distribution process, whether raw materials, in-process

inventories, or finished products.

2. Personnel. This generally refers to actual people, as opposed, for example,

to hours of labor.3. Capital equipment. This includes such things as factory space, tools, and

other equipment necessary for the production of goods and provision of

services.

4. Orders. This includes such things as orders for goods, requisitions for new

employees, and contracts for new space or capital equipment. Orders aretypically the result of some management decision which has been made, but

not yet converted into the desired result.

5. Money. This is used in the cash sense. That is, a flow of money is the actual

transmittal of payments between different stocks of money.

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Information

Stocks, flows, information

The information links in a business process canbe difficult to adequately model because of the

abstract nature of these links.Materials, personnel, capital equipment, orders,and money usually have a physicalrepresentation. Furthermore, these quantities are

conserved, and thus they can only flow to oneplace at a time.Information, on the other hand, cansimultaneously flow to many places, and,

particularly in computer-intensive environments,it can do this ra idl and with considerable

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Fundamental Structures and Behaviors

General Modelling

Approach

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Fundamental Structure &Behaviours

1. Growth

2. Decay

3. S-shape4. Osccilation

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1. Exponential Growth

Savings (sudah dibahas)

a first order positive feedback loop

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2. Exponential Decay

worker

Suppose that you have 100 people working for you and you

decide never to hire anyone again. Your average worker

hangs around for 10 years.

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3. S-shape

mice

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EQUATIONS:


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(01) effect of density on survival=

effect of density on survivalfunction(Mice/normal carrying

capacity)Units: Dmnl

(02) effect of density on survivalfunction(

[(0,0)-(4,3)],(0,3),

(1,1),(2,0.25),(4,0))Units: Dmnl

(03) FINAL TIME = 60

Units: Month

The final time for the simulation.

(04) INITIAL TIME = 0

Units: Month

The initial time for the simulation.

(05) max net new mice rate=

0.25

(06) Mice= INTEG (

net new mice,

2)

Units: Mice

(07) net new mice=

max net new mice rate * Mice *

effect of density on survival

Units: Mice/Month

(08) normal carrying capacity=

200Units: Mice

(09) SAVEPER =

TIME STEP

Units: Month

The frequency with which output is stored.

(10) TIME STEP = 0.25Units: Month

The time step for the simulation.

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4. Oscillation

Range-kutta

In a complex system, thereare likely to be a number of

minor negative loops and

these, in general, will be

damping

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l4-sd stock flow

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