**Mental model**

A decision-making process we conduct in our heads.

**Visual model**

A model in which graphics or diagrams are used to convey real objects or situations. Examples are a map or a graph.

A number of different types of models exist. The most common are **mental models**, which we “build” in our heads and use to make decisions. The traffic-light situation calls for a mental model. **Visual models** use graphics or diagrams to represent real objects or situations. For example, a road atlas represents a system of roads and other key land features. **Physical models** involve objects that represent other objects, such as an architect's scale model of a new building. **Mathematical models** use equations and relationships among quantities to represent situations. Many of the concepts in this textbook are shown through the use of mathematical models. **Spreadsheet models** are a means of implementing mathematical models.

Although there are a number of different model types, commonalities exist among them. First, the use of models is motivated by a *decision* that needs to be made, for example, whether to stop or go at a yellow light or how much of a product to order at one time. Second, all models rely on inputs. **Inputs** are quantities or factors that affect the situation. Inputs can be controllable ...

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