2.6. Questions and Tasks
41
momentum; any continuity requirements. Also, check for consistency of
units. Look for the possibilities of division by zero , negative volumes
or prices, etc. Use conditional statements if necessary to avoid these
violations .
6. To see how the model is going to work , choose some time horizon
over which you intend to examine the dynamic behavior of the model ,
the length of each time interval for which state variables are being updated, and the numerical computation procedure by which flows are
calculated. (For example, choose DT= 1, time length = 24.) Set up a
graph and make an educated guess about the variation of the state variable curves before running the model.
7. Run the model. See whether the graph of these variables passes a "sanity test." Choose alternative length s of each time interval for which state
variables are updated. Choose alternative integration techniques. (For
example, reduce in STELLA the time interval DT by half and run the
model again to see if the results are the same.)
8. Vary the parameters to their reasonable extremes and see whether the
results in the graph still make sense . Revise the model to repair errors
and anomalies.
9. Compare the results with experimental or census data . This may mean
shutting off parts of your model to mimic a lab experiment, for
example.
10. Revise the parameters, perh aps even the model to reflect greater complexity and to meet exceptions to the experimental results, repeating
steps 1-10. Frame a new set of interesting que stions .
Don 't worry 'about applyin g all of these steps in this order as you develop your model s and improve your modeling skills. However, refer to
this list now and then to see how useful, inclusive and reasonable these
steps are .
Remember that modeling has three possible general uses. First, you can
experiment with models . A good model of a system enables you to change
its components to see how these changes affect the rest of the system. Second, a good model help s you explore likely future courses of a dynamic
system. Third, a good model stimulates further que stions about the system
behavior and the applicability of the principle s that are discovered in the
modeling proce ss to other systems.
2.6. Questions and Tasks
1. Modify the graphical relationship between whale population size and reproduction rate. Alternatively choose higher or lower upper bounds on
reproduction rates and observe the implications of your choices for
model results. Plot your results in a graph and in a table and comp are
41
momentum; any continuity requirements. Also, check for consistency of
units. Look for the possibilities of division by zero , negative volumes
or prices, etc. Use conditional statements if necessary to avoid these
violations .
6. To see how the model is going to work , choose some time horizon
over which you intend to examine the dynamic behavior of the model ,
the length of each time interval for which state variables are being updated, and the numerical computation procedure by which flows are
calculated. (For example, choose DT= 1, time length = 24.) Set up a
graph and make an educated guess about the variation of the state variable curves before running the model.
7. Run the model. See whether the graph of these variables passes a "sanity test." Choose alternative length s of each time interval for which state
variables are updated. Choose alternative integration techniques. (For
example, reduce in STELLA the time interval DT by half and run the
model again to see if the results are the same.)
8. Vary the parameters to their reasonable extremes and see whether the
results in the graph still make sense . Revise the model to repair errors
and anomalies.
9. Compare the results with experimental or census data . This may mean
shutting off parts of your model to mimic a lab experiment, for
example.
10. Revise the parameters, perh aps even the model to reflect greater complexity and to meet exceptions to the experimental results, repeating
steps 1-10. Frame a new set of interesting que stions .
Don 't worry 'about applyin g all of these steps in this order as you develop your model s and improve your modeling skills. However, refer to
this list now and then to see how useful, inclusive and reasonable these
steps are .
Remember that modeling has three possible general uses. First, you can
experiment with models . A good model of a system enables you to change
its components to see how these changes affect the rest of the system. Second, a good model help s you explore likely future courses of a dynamic
system. Third, a good model stimulates further que stions about the system
behavior and the applicability of the principle s that are discovered in the
modeling proce ss to other systems.
2.6. Questions and Tasks
1. Modify the graphical relationship between whale population size and reproduction rate. Alternatively choose higher or lower upper bounds on
reproduction rates and observe the implications of your choices for
model results. Plot your results in a graph and in a table and comp are