# Lesson 8 - Unit 2 Summary

In Unit 2, we very briefly covered the basic concepts upon which GoldSim models are based:

• Simulation is the process of creating a model of an existing or proposed system in order to identify and understand the factors that control the system, or to predict the future behavior of the system. In a dynamic simulation, the system model changes and evolves with time.

• Deterministic simulation often represents "the best guess" or "worst case" values.

• Probabilistic simulation represents uncertainty by specifying inputs as probability distributions. As such, it represents uncertainty explicitly and quantitatively. If the inputs to a model are uncertain, the outputs are necessarily uncertain.

• Simulation is a powerful and important tool because it provides a way in which alternative designs, plans and/or policies can be evaluated without having to experiment on a real system.

• GoldSim is a program for carrying out probabilistic, dynamic simulations of complex systems. It is essentially a high-level programming language for building simulation models (but does not require you to be a computer programmer).

• GoldSim represents parameters, processes, or events in a system using objects called elements. Each element has a symbol or graphical image to represent it. An element accepts one or more inputs and produces one or more outputs. There are six primary categories of elements: Inputs, Functions, Events, Stocks, Delays, and Results.

• GoldSim represents the links (dependencies) between elements using arrows called influences. GoldSim ensures dimensional consistency and carries out all unit conversions for you.

• A special type of element, the Container, is used to hierarchically organize other elements, allowing you to build and maintain large, complex models.

• In order to simulate how a system might evolve over time in a program like GoldSim, it is necessary to discretize time into discrete intervals referred to as timesteps. GoldSim then “steps through time” by carrying out calculations every timestep, with the values at the current timestep often computed as a function of the values at the previous timestep.

• After running a model, GoldSim can generate and display different types of results, in either graphical or tabular form. The most common results viewed are time history results and distribution results. A time history result simply shows how a model output is predicted to change with time. As such, it is the fundamental type of result produced by a dynamic simulation model. A distribution result is the fundamental type of result produced by a probabilistic simulation model. It shows a probability distribution of an output at a specific point in time (e.g., the end of the simulation).

If you are new to simulation, you likely have many questions at this point. However, the basic information provided in this Unit provides the necessary background information required for us to start answering those questions in the next Unit.  We will do so through some “hands-on” work with GoldSim.