Tutorial: Reviewing a model

Analytica TutorialTutorial: Reviewing a model

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This Tutorial page shows you how to explore a model by examining the Rent vs. Buy model's:

Influence diagrams
Variables
Attributes
Definitions
Results

This assumes you have started Analytica and have opened the Rent vs. Buy model. If this is not the case, see Opening the Rent vs. Buy model. If you are using the model as modified from Chapter 1, change the value of Time horizon back to 10, the value of Monthly rent back to 1200, and the value of Buying price back to 140K. Also change the Rate of inflation back to a normal distribution with a mean of 3.5 and a standard deviation of 1.3.

Tutorial Video: Review/explore a model (8 minutes)

Recognize influence diagrams

In this chapter, you will delve into some of the details of the Rent vs. Buy model. You will not use the top diagram that you used in Chapter 1.

1. Double-click the Model node to open it.

The details of an Analytica model display in an influence diagram window. An influence diagram (shown below) is a graphical representation of a model, showing how different variables in the model interact with each other. A typical influence diagram consists of a number of nodes connected by arrows.

Tools palette⠀

Nodes⠀

Influence diagram window

Nodes represent variables and appear as boxes, ovals, hexagons, and other shapes. Different node shapes represent different types of variables. Analytica uses the term variable broadly to include anything that has a value or can be evaluated. Note that many of the variables have the same names as the inputs and output at the top diagram that you used in Chapter 1. The top diagram provides an easy way to see and change these nodes’ values.

Arrows connecting different variables indicate a relation between the variables. The arrow connecting Rate of inflation to Appreciation rate indicates that the value of the Appreciation rate variable depends on the value of the Rate of inflation variable. In the Rent vs. Buy model influence diagram, Cost to Buy depends on the Buying price, Rate of inflation, Appreciation rate, Discount rate, and Time horizon variables.

The following figure illustrates different types of nodes.

Open Object windows

Every object in Analytica has an associated Object window containing detailed information about it. You can display the Object window of any variable by double-clicking its node in the influence diagram.

1. Double-click the Buying price node to open the Buying price Object window.

Information about a variable is provided in a list of attributes. Attributes include the variable’s class (for example, decision, chance, or constant), identifier, units, title, description, definition, inputs, and outputs. See the illustration below.

Tip

You can enter numbers with a suffix abbreviation, so Buying Price can be defined as either 140K or 140000. A quick reference for these suffixes is given on Number formats page.

Move between Object windows

You have opened the Object window of a variable (Buying price) by double-clicking its node in the influence diagram.

The Object window contains a list of the variable’s inputs and outputs, if there are any.

You can open the Object window for any input or output variable by double-clicking the one you wish to view.

1. Double-click the output variable titled Mortgage loan amount.

Analytica opens the Object window for Mortgage loan amount.

Note in the figure above that the Title of Mortgage loan amount is different from the variable’s identifier, Mortgage. The title is what the model user normally sees; the identifier is used as a mathematical symbol in the definitions of other variables that depend on this variable.

The definition of the Mortgage loan amount is an expression, the sum of Buying price and Down payment (which is a negative amount). The definition refers to these variables by their identifiers.

Tip

The color of the text in the definition varies, to help you spot mistakes as you read or edit Definitions. See Syntax coloring in the User Guide.

Inputs lists the identifiers and titles of the variables in the definition. Buying price, the variable you just examined, is one of the inputs. The other input of Mortgage loan amount is Down payment.

2. Double-click the Down payment input.

The Object window now displays the attributes of Down payment.

3. Click the close button to close the window and return to the diagram.

Use the Attribute panel

As an alternative to viewing a variable’s attributes in a separate window, you can inspect them in the Attribute panel, which is an auxiliary window pane that you can open below the influence diagram.

The Attribute panel allows you to rapidly examine one attribute at a time of any variable in the model. You select the variable you wish to view and select the attribute to examine from a popup menu.

The variable Buying price should be highlighted with a title in white, indicating that it is selected; if it is not, select it by clicking it once.

1. Click the down arrow icon to open the Attribute panel.

By default, Analytica displays the description of the selected node (e.g., Buying price) in the Attribute panel.

Inspect definitions in the Attribute panel

The Attribute panel allows you to inspect any attribute of a variable.

In this section, you will see the definition of two variables that you viewed in the top-level diagram in Chapter 1.

1. Click Description to view the Attribute popup menu.

2. Select Definition as the new current attribute to display.

3. The definition of Buying price is displayed. It is a single number: 140K (140,000).

4. Select the Rate of inflation node.

The definition in this example is a normal distribution; hence, the Normal function is shown.

In Chapter 1 you saw that Rate of inflation is defined as a normal distribution with a mean of 3.5 and a standard deviation of 1.3.

Open modules

Analytica models generally contain modules. Each module contains the details of a part of the model, also represented as an influence diagram. In the Rent vs. Buy model, Cost to Buy and Cost to Rent are both modules.

Modules can also contain other modules. In this manner, a large model with hundreds of variables can be organized into a hierarchy of modules, each small enough to be easily understood.

1. Double-click the Cost to Buy node to open the module.

Analytica displays the influence diagram of the Cost to Buy module. This module contains three additional modules: Out-of-pocket costs to own, Future sales proceeds, and Opportunity cost.

Output arrowhead⠀

Input arrowhead⠀

The input arrowhead (without a trailing line) shows that the node to the right of the arrow has one or more inputs from outside this module.

The output arrowhead shows that the node to the left of the arrow has one or more outputs outside this module.

2. Click the Diagram button to return to the parent diagram, Model.

3. Double-click the Cost to Rent node to open the module.

The Cost to Rent diagram opens (see the figure below).

Analytica limits the number of open windows at each level of the model hierarchy to minimize clutter on your screen. See Managing windows in the Analytica User Guide for information on how to open more than one module Diagram window at a time.

4. Click the Diagram button to return to the parent diagram, Model.

Combined arrowhead⠀

The combined arrowhead, shown above, indicates that the node has one or more inputs from outside this module, plus the input variable in this module.

You can also navigate the model by tracking a variable’s inputs or outputs.

5. Select the Buying price node.

6. Select Outputs from the Attribute popup menu to view a list of variables that depend on the Buying price variable.

7. Double-click Insurance.

The Out-of-pocket costs to own module diagram is brought to the front, with the Insurance node selected.

Learn about nodes using Help balloons

Help balloons pop up when you hover over a node for about a second, and display the title, units, and description for a variable. These allow you to browse information very rapidly without having to open the object window. Balloons do not appear when objects do not have the Help or Description attribute filled in.

8. Hover the mouse over the node and wait for about 1 second.

9. Click the Diagram button to view the Cost to buy diagram.

Using Pop-up node controls

Pop-up node controls appear around a node, when you move the mouse cursor over a node and wait half a second, or when you click a node. Click on a pop-up control to open the Object Window, Result window, or Expression view for this Variable, or to open the Diagram for a Module node. See the User guide for a complete explanation Pop-up_node_controls.

10. When you Hover the mouse over a node Popup node controls appear - usually before the help balloon (if present).

10. When you Hover the mouse over the node Popup node controls appear - usually before the help balloon (if present).

Inspect values in the Attribute panel

The Attribute panel allows you to view certain attributes, such as a variable’s value, that are not (initially) displayed in an Object window.

1. Select the Present value of buying node.

2. Click the key icon to open the Attribute panel.

3. Press the Attribute popup menu and select Value.

The deterministic (or mid) value of Present value of buying displays, in this case, -67.2K.

If Value was not previously computed, Analytica computes the variable’s value deterministically, assuming that all of the input probability distributions are fixed at their median values. Mid value is an abbreviation for this deterministically computed value.

You can use the Attribute panel in this manner to examine the mid value of any variable in the model.

It is faster to compute a mid (deterministic) value than an uncertain (probabilistic) value, so it is useful for conducting initial checks of a model before performing any uncertainty analysis.

Display results

When you are viewing a model’s influence diagram, you can evaluate any variable and display its value in a Result window.

1. With Present value of buying still selected, click the Result button to evaluate it.

A Result window displays the probability density function graph for this variable. Analytica displays the uncertainty view that was most recently selected from the Uncertainty View popup menu, or that was saved with the model.

2. Click the close button to close the Result window.

Tip

The Rent vs. Buy model uses financial flow conventions: funds flowing in (received) have positive values; funds flowing out (expended) have negative values.

As an alternative to clicking the Result button and then selecting an uncertainty view, you can use the Result menu to evaluate a variable and select the uncertainty view of the result.

3. With Present value of buying still selected, select the Result menu. The check mark next to Probability Density indicates that the Probability Density was last displayed. Select Cumulative Probability.

The Result window appears displaying the variable’s cumulative probability distribution.

4. Click the Diagram button to display the Cost to buy diagram.

5. Select Moving costs and then click the Result button to evaluate.

A single mid value appears in table view.

Uncertainty View Popup Menu⠀

The mid value in table view is the only result view available for a nonprobabilistic variable with a single value.

6. Select Probability Mass from the Uncertainty View popup menu.

Analytica tells you that this is a nonprobabilistic value.

7. Click the OK button.

Summary

You now have browsed the Rent vs. Buy model by examining its influence diagrams, variables, attributes, definitions, and results. These are the basic techniques for exploring any Analytica model.

The next chapter shows you how to analyze the Rent vs. Buy model.

You can quit Analytica at this point.