26
2. Modeling in STELLA
of the stock of whales through time match your expectations? If not, explore
where you went wrong in your reasoning.
How does STELLA determine the time path of the state variable? Here is
a brief description; a more rigorous explanation of the different ways in
which STELLA can and does calculate model results is provided in Section
2.3 below. At the beginning of each time period, starting with time = 0
years (the initial period), STELLA looks at all the components for the required calculations. The values of the state variables will form the basis for
these calculations. Only the variable REPRODUCTION depends on the state
variable WHALES. To estimate the value of REPRODUCTION after the first
time period, STELLA multiplies 0.05 by the value WHALES (at time = 0) or
200 (provided by the information arrows) to arrive at 10. To update the
stocks every quarter of a period, STELLA calculates:
WHALES(t) =WHALESCt-DT) + (REPRODUCTION)"DT
0)
where DT= .25. STELLA repeats the process of updating the state variable
each DT throughout the length of the model run. When you plot your
model results in a table, you find that, for our simple whale population
model, STELLA calculates fractions of whales . This problem is easy to solve,
for example, by having STELLA round the calculated number of whales ,
with a built-in function that can do that, or just by reinterpreting the population size as being measured in thousands of whales .
This process of calculating stocks from flows highlights the important
role played by the state variable. The computer carries that information,
and only that information, from one DT to the next , which is why it is defined as the variable that represents the condition of the system.
You can "drill down" in the STELLA model to see the parameters and
equations that you have specified and how STELLA makes use of them .
Click on the downward-pointing arrow at the left of your STELLA diagram.
The equations of the model developed in this section are listed there . To
ease model development, we show STELLA equations of each model at the
end of the chapter or section in which it has been developed.
BASIC POPULATION MODEL
WHALES(t) = WHALES(t - dt) + (REPRODUCTION) * dt
INIT WHALES = 200
INFLOWS:
REPRODUCTION = REPRODUCTION_RATE*WHALES
REPRODUCTION_RATE = .05
2. Modeling in STELLA
of the stock of whales through time match your expectations? If not, explore
where you went wrong in your reasoning.
How does STELLA determine the time path of the state variable? Here is
a brief description; a more rigorous explanation of the different ways in
which STELLA can and does calculate model results is provided in Section
2.3 below. At the beginning of each time period, starting with time = 0
years (the initial period), STELLA looks at all the components for the required calculations. The values of the state variables will form the basis for
these calculations. Only the variable REPRODUCTION depends on the state
variable WHALES. To estimate the value of REPRODUCTION after the first
time period, STELLA multiplies 0.05 by the value WHALES (at time = 0) or
200 (provided by the information arrows) to arrive at 10. To update the
stocks every quarter of a period, STELLA calculates:
WHALES(t) =WHALESCt-DT) + (REPRODUCTION)"DT
0)
where DT= .25. STELLA repeats the process of updating the state variable
each DT throughout the length of the model run. When you plot your
model results in a table, you find that, for our simple whale population
model, STELLA calculates fractions of whales . This problem is easy to solve,
for example, by having STELLA round the calculated number of whales ,
with a built-in function that can do that, or just by reinterpreting the population size as being measured in thousands of whales .
This process of calculating stocks from flows highlights the important
role played by the state variable. The computer carries that information,
and only that information, from one DT to the next , which is why it is defined as the variable that represents the condition of the system.
You can "drill down" in the STELLA model to see the parameters and
equations that you have specified and how STELLA makes use of them .
Click on the downward-pointing arrow at the left of your STELLA diagram.
The equations of the model developed in this section are listed there . To
ease model development, we show STELLA equations of each model at the
end of the chapter or section in which it has been developed.
BASIC POPULATION MODEL
WHALES(t) = WHALES(t - dt) + (REPRODUCTION) * dt
INIT WHALES = 200
INFLOWS:
REPRODUCTION = REPRODUCTION_RATE*WHALES
REPRODUCTION_RATE = .05