Lesson 1 - Unit 14 Overview

"The ability to define what may happen in the future and to choose among alternatives lies at the heart of contemporary societies."

- Peter Bernstein

Back in Unit 2, we discussed why you would want to carry out a simulation. We noted that the primary objective of building simulation models was to better understand the controlling factors and processes in the system and use this information to predict (forecast) the future behavior of the system under various sets of assumptions.

The primary reason we would do so is so we can determine what we can do to influence the future behavior of the system.  That is, our goal is often to use the simulation model to identify any necessary changes we can make to the system that will help make it perform the way that we want it to. Such changes range from changing the physical design of the system (e.g., increasing the number of scanning stations at a security checkpoint) to changing policies for how the system is run (e.g., changing the decision rules for opening and closing new stations).

Hence, simulation provides a mechanism by which alternative designs, plans, and/or policies can be evaluated without having to experiment on a real system, which may be prohibitively costly, time-consuming, or simply impractical to do.  That is, it allows you to ask "What if?" questions about a “virtual” system without having to experiment on the actual system.

To accomplish this in GoldSim (or any simulation tool), what we want to do is: 1) build a simulation model; 2) run it multiple times using different inputs (representing different designs or policies); and then 3) compare the results of those different simulation runs to determine which set of inputs best meets our goals for the system.

In the last 13 Units we have learned how to build a model, change the inputs, run the model, and look at results.  So conceptually, we have everything we need to carry this out and ask these “What if?” questions about the system we are modeling.  Practically, however, doing so with the tools we have discussed can be difficult.  Why?  Because every set of inputs (and corresponding results) must be stored in a separate model file.  This makes comparing different results difficult. Using what we have learned so far, in order to compare the results of different runs you would need to export the key results (e.g., to a spreadsheet) and compare them there.  Moreover, since the different runs would be represented by different model files, there is no easy way to determine how the models are different (i.e., what are the differences in the inputs?).

To address this, GoldSim provides a powerful and flexible feature referred to as scenario modeling. GoldSim’s scenario modeling capability allows you to directly compare results generated by your model using different sets of input parameters.  In effect, when you use this capability, your model can store (and subsequently compare) multiple sets of inputs and outputs.

This Unit discusses this important feature.

In particular, this Unit includes the following Lessons:

• Introduction to scenario modeling;
• Creating scenarios;
• Understanding the Active Scenario;
• Running scenarios;
• Comparing scenario results;
• Editing scenarios and adding new scenarios; and
• Changing model logic, Time Series and other inputs between scenarios.

This Unit also includes one Exercise (in addition to several Examples that we will work through together). This Unit has a total of 10 Lessons (including this overview and a summary at the end).