Classes of variables and other objects

Analytica User GuideExamining a ModelClasses of variables and other objects

The shape of a node indicates the class of the variable or other object:

	A rectangle depicts a decision variable — a quantity that the decision maker can control directly. For example, whether or not you take an umbrella to work is your decision. If you are bidding on a contract, it is your decision how much to bid.
	An oval depicts a chance variable — that is an uncertain quantity whose definition contains a probability distribution. For example, whether or not it will rain tomorrow is a chance variable (unless you are a rain god). And whether or not your bid is the winning bid is a chance variable in your model, although it is a decision variable for the person or organization requesting the bid. See Expressing Uncertainty for how to define a chance variable.
	A hexagon depicts an objective variable — a quantity that evaluates the relative value, desirability, or utility of possible outcomes. In a decision model, you are trying to find the decision(s) that maximize (or minimize) the value of this node. Usually, a model contains only one objective.
	A rounded shape (with thin outline) depicts a general variable — a quantity that is not one of the above classes. It can be uncertain because it depends on one or more chance variables. Use this class initially if you’re not sure what kind of variable you want. You can change the class later when it becomes clearer.
	An hourglass shape depicts a constraint — a relationship utilized when solving constrained optimization problems in the Analytica Optimizer edition. The constraint node appears on the toolbar only when using Analytica Optimizer. Optimization is covered in the Optimizer user guide.
	A rounded node (with thick outline) depicts a module — that is, a collection of nodes organized as a diagram. Modules can themselves contain modules, creating a nested hierarchy.
	A parallelogram depicts an index variable. An index is used to define a dimension of an array. For example, Year is an index for an array containing the U.S. GNP for the past 20 years. Or Nation name is an index for an array of GNPs for a collection of nations. Indexes identify the row and column headers of a table, and the axes and key of a graph (see Introducing indexes and arrays.
	A trapezoid depicts a constant — that is, a variable whose value is fixed. A constant is not dependent on other variables, so it has no inputs. Examples of numerical constants are the atomic weight of oxygen (16) or the number of feet in a kilometer. It is clearer to define a constant for each such value you need in a model, so you can refer to them by name in each definition that uses it, rather than retyping the number each time.
	A shape like an arrow tail depicts a function. You can use existing functions from libraries, and define new functions to augment the functions provided in Analytica. See User-defined Functions and Libraries.
	This node is a button — when you click a button (in browse mode), it executes its script to per- form some useful action. You can use buttons with any edition of Analytica, but you need Analytica Enterprise or Optimizer to create a new button (see Creating buttons).

Module Subclasses

All modules contain other objects, including sometimes other modules. There are several different subclasses of module:

Model: Usually the top module in a module hierarchy, saved as a separate file (document with extension .ana). Any nondefault preferences (see Preferences dialog, uncertainty options (see Uncertainty Setup dialog), and graph style templates are saved with the model, but not other module types.

Module: A collection of nodes displayed in a single diagram. A standard module contains a set of other nodes, and is usually part of the module hierarchy within a model or other module type.

Filed module: A module whose contents are saved in a file separate from the model that con- tains it. A filed module can be shared among several models, without having to make a copy for each model.

Library: A module that contains functions and sometimes variables. Read-in libraries are listed in the Definition menu below the built-in libraries, with a hierarchical submenu listing the functions they contain, giving easy access.

Filed library: A library saved in a file separate from the model that contains it. A filed library can be shared among several models, without having to make a copy for each model.

Form: A module containing input and output nodes. You can easily create input and output nodes in a form node by drawing arrows from their original node to the form (for outputs) or from the form to the variable for inputs. See Creating Interfaces for End Users.

You can change a module from one of these subclasses to another -- for example, to convert a standard module to a separately filed module. See below for how to do that.

Classes of variables and other objects

Module Subclasses

See Also