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2. Modeling in STELLA
To do this, click on the Text symbol (the letter A) and drag it into the diagram. Then, type in your annotation.
The tools we mentioned here are very likely to prove useful when you
develop more complicated models and when you want to share your models and their results with others . STELLA contains very helpful tools, which
we hope you will use extensively. Space limitations preclude us from describing all of STELLA's features . The Appendix provides a brief overview
of STELLA features .
Make thorough use of your model , running it over again and always
checking your expectations against its results. Change the initial conditions
and try running the model to its extremes. At some point you will want to
carry out a formal sensitivity analysis. We will discuss sensitivity analysis in
Chapter 3. First, however, we wish to discuss the numeric solutions methods that STELLA uses to calculate model results. The choice of solution
method may have some bearing on the accuracy of your results.
BASIC POPULATION MODEL WITH ENDOGENOUS
REPRODUCTION RATE
WHALES(t) = WHALES (t-dt) + (REPRODUCTION-MORTALITY) *
dt
INIT WHALES = 200
INFLOWS;
REPRODUCTION = REPRODUCTION_RATE*WHALES
OUTFLOWS :
MORTALITY = MORTALITY_RATE*WHALES
MORTALITY_RATE = .03
REPRODUCTION_RATE = GRAPH(WHALES)
(2.00, 0.08), (61.8, 0.072),
0 .0588), (241, 0.046), (301,
(421, 0.0112), (480, 0 .006),
(122, 0 .066), (181,
0.0264), (361, 0 .0172),
(540, 0.002), (600, 0.00)
2.3. STELLA's Numeric Solution Techniques
This section provides a brief description of, and basic mathematical background information on, the numeric techniques available in STELLA to
solve the equations that define a model. Knowing about these techniques
will be important in understanding how the computer arrives at model results, the accuracy of your results, and methods to improve accuracy.
From the models above we have seen that STELLA calculates the value of
a stock in a given time period t based on the value of that stock a time step
earlier plus the net of the inflows and outflows that occur over that time
2. Modeling in STELLA
To do this, click on the Text symbol (the letter A) and drag it into the diagram. Then, type in your annotation.
The tools we mentioned here are very likely to prove useful when you
develop more complicated models and when you want to share your models and their results with others . STELLA contains very helpful tools, which
we hope you will use extensively. Space limitations preclude us from describing all of STELLA's features . The Appendix provides a brief overview
of STELLA features .
Make thorough use of your model , running it over again and always
checking your expectations against its results. Change the initial conditions
and try running the model to its extremes. At some point you will want to
carry out a formal sensitivity analysis. We will discuss sensitivity analysis in
Chapter 3. First, however, we wish to discuss the numeric solutions methods that STELLA uses to calculate model results. The choice of solution
method may have some bearing on the accuracy of your results.
BASIC POPULATION MODEL WITH ENDOGENOUS
REPRODUCTION RATE
WHALES(t) = WHALES (t-dt) + (REPRODUCTION-MORTALITY) *
dt
INIT WHALES = 200
INFLOWS;
REPRODUCTION = REPRODUCTION_RATE*WHALES
OUTFLOWS :
MORTALITY = MORTALITY_RATE*WHALES
MORTALITY_RATE = .03
REPRODUCTION_RATE = GRAPH(WHALES)
(2.00, 0.08), (61.8, 0.072),
0 .0588), (241, 0.046), (301,
(421, 0.0112), (480, 0 .006),
(122, 0 .066), (181,
0.0264), (361, 0 .0172),
(540, 0.002), (600, 0.00)
2.3. STELLA's Numeric Solution Techniques
This section provides a brief description of, and basic mathematical background information on, the numeric techniques available in STELLA to
solve the equations that define a model. Knowing about these techniques
will be important in understanding how the computer arrives at model results, the accuracy of your results, and methods to improve accuracy.
From the models above we have seen that STELLA calculates the value of
a stock in a given time period t based on the value of that stock a time step
earlier plus the net of the inflows and outflows that occur over that time