L5-SD_Stock_Flow.ppt

7/27/2019 L5-SD_Stock_Flow.ppt

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Stocks & Flows Diagrams:

EXTENDED

Based on

Budi HARTONO

Sugeng Purwoko


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Adding Input / Output Objects

Slider Bars for constant-valued auxiliary variables

Slider Bars to set the initial value of box variables

Making a switch out of a Slider

Adding custom graphs/tables to the build window

Sugeng Purwoko


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REFERENCE CASE THE FLU

Susceptible People is initially 99

Infected People is initially 1

Infection Fraction = 0.0151

Get Sick = Susceptible People *

Infected People * Infection Fraction Model Settings: Initial Time = 1, Final

Time = 10, TimeStep = 0.1, unit for time

= days

Sugeng Purwoko


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Slider Bars for Constant-Valued Auxiliary Variables

Changing the infection fraction

Why?

What-if analysis

Calibration

Sugeng Purwoko


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Procedure1.

2.

3.

Sugeng Purwoko


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Slider Bars to Set the Initial Value of a Box Variable

to vary the initial value ofSusceptible People.

Sugeng Purwoko


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Procedure

Add a new auxiliary variable and name it Initial Valueof Susceptible People.

Add a connector from this auxiliary variable toSusceptible People.

Open the equations window ofSusceptible People. Set its initial value equal to Initial Value of Susceptible

People.

Now open the equations window for Initial Value of

Susceptible People and specify a value. Create a slider for your auxiliary variable as described

earlier

Sugeng Purwoko


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Making Switches Out of Sliders

restricting them to a value of either 0 or 1

turning them into On/Off switches for rates

When Get Sick On/Off = 0, the formula for Get Sick

evaluates to 0:

GS = S * I * k * 0 = 0.When Get Sick On/Off = 1, the formula for Get Sick

is normal:

GS = S * I *k * 1 = S * I * k.

GS = S * I * k * o

Sugeng Purwoko


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GS = S * I * k * o

Sugeng Purwoko


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Adding Custom Graphs and Tables to the Build Window

Sugeng Purwoko


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Case 4 The Workforce-Inventory Example

Self-exercise

Sugeng Purwoko


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The Context

You are involved in the production and sale of prefabricated

window frames. Overall your company is doing quite well, but

you often go through periods of low capacity utilization

followed by production ramp up and added shifts.

While all of this is normally blamed on market demand and

the condition of the economy, you have your doubts.

Looking back at sales and production over the last 8 years it

seems that sales is more stable than production.

Your goal is to determine why this might be, and what you can

do about it.

Sugeng Purwoko


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Approach

In attacking this problem you want to simplify as much as

possible your current situation. There are a number of

reasons for this simplification:

It is easier to understand a simple model.

You can get results quickly and decide if you are on the right track.

It is more effective to start with a simple model and add detail, than to build a

complex model and attempt to extract insights from it after it is complete.

Using a simple model forces you to take an overview which is usually useful in

the initial modeling phases.

Sugeng Purwoko


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The Iceberg: events patterns - structure

Sugeng Purwoko


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Dynamic Hypothesis

an idea about what structure might be capable ofgenerating behavior like that in the reference modes.

Key variables of interest:

____

____

The dynamic hypothesis for this firm is that a manager issetting production based on current sales, but isamplifying the amount resulting in higher (or lower)production than is necessary.

Sugeng Purwoko


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Sales Production

Related by:

physical : production is required to produce goods to

sell

Information: managers base production decisions oncurrent or recent sales

Sugeng Purwoko


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Physical Relationships

Stocks?

Flows?

Sugeng Purwoko


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Workforce

Sugeng Purwoko


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Behavioral Relationships

information connections.

a stock adjustment process

Sugeng Purwoko


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Stock Adjustment Process

Sugeng Purwoko


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Sugeng Purwoko


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Equation Set

FINAL TIME = 100

Units: Month

INITIAL TIME = 0

Units: Month

TIME STEP = 0.25

Units: Month

SAVEPER = TIME STEP

Units: Month

Sugeng Purwoko


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Inventory = INTEG(production-sales, 300)

Units: Frame

net hire rate=(target workforce-Workforce)/time to adjust workforceUnits: Person/Month

production= Workforce*productivity

Units: Frame/Month

productivity= 1

Units: Frame/Month/Person

sales= 100 + STEP(50,20)

Units: Frame/Month

target production= sales

Units: Frame/Month Sugeng Purwoko


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target workforce= target production/productivity

Units: Person

time to adjust workforce= 3

Units: Month

Workforce = INTEG(net hire rate, target workforce)

Units: Person

Sugeng Purwoko


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Run Simulation & analysis

VS.

Sugeng Purwoko


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Sugeng Purwoko


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One missing loop is introduced

Sugeng Purwoko


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Added equation

target production = sales + inventory correction

Units: Frame/Month

inventory correction = (target inventory - Inventory)/

time to correct inventory

Units: Frame/Monthtime to correct inventory = 2

Units: Month

target inventory = sales * INVENTORY COVERAGE

Units: Frame

INVENTORY COVERAGE = 3

Units: Month

Sugeng Purwoko


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Sugeng Purwoko


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Sugeng Purwoko


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Sugeng Purwoko


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inventory

Target inv

sales

production

Sugeng Purwoko


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Policy Analysis - Sensitivity

More aggressive correction for inventory deviations

Time to correct inventory: 1 month instead of 2 months

Sugeng Purwoko


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Reality Check model verification

Extreme condition test

"Without Inventory we can't ship."

Sugeng Purwoko

Documents

L5-SD_Stock_Flow.ppt