Dynamic systems are all around us - they pervade our lives in
addition to the study of the Earth - and many of the pressing
questions and problems that we face require a good understanding of
how these systems work; how they react to changes.

In this chapter, we have looked at a few simple examples of
dynamic systems and have seen how they can be modeled using the
program STELLA. By working through the introductory example, you
should have acquired a beginning proficiency with the program, which
will be used extensively throughout this book. The primary reason for
modeling these systems using a computer is that many of them are so
complex and so unwieldy as to make experimentation impossible and
their dynamics cannot generally be understood without
experimentation. STELLA provides a powerful, versatile, and yet
user-friendly means for modeling these dynamic systems on a
computer.

This chapter has also introduced some of the more common types of
system designs and the kinds of behaviors that result from these
designs. These basic system structures will occur again and again in
a variety of contexts.

One of the major challenges in computer modeling is to understand
at what level the results should be interpreted. Should they be taken
as precise simulations and representations of the real world? Should
their results be considered in a qualitative way only? Should their
results be considered only in an abstract sense, without any
connection to the real world? Are the results meaningless since the
full complexity of nature can never be included in a model? These
questions get to the very heart of modeling. Models may be
constructed with different intents; some are designed to generate
realistic results that have predictive value, while others are merely
designed to investigate the implications of some assumption. It is
always important to understand what the intent of the model is first
- that gives us a clue as to how to interpret the results. Models
that attempt to be realistic must be compared with observations of
the dynamics of the real-world system.

In almost all cases, making a model requires a good sense for what
are appropriate and acceptable simplifications. The water tub system
provided a clear example of a range of models that helped to
illustrate the difference between "as simple as possible" "not too
simple".

With some sense of how to interpret the results of these models,
you are prepared to move on and actively explore some fascinating and
important aspects of the dynamic system we call Earth.

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