Defining Decision Variable Cells

Decision variables are the variables that you can control, such as rent to charge or the amount of money to invest. Decision variables are not required for simulation models, but they can be useful when comparing and optimizing alternate scenarios. Several of the Crystal Ball tools discussed in Crystal Ball Tools, use and benefit from decision variables.

You also use decision variables with OptQuest, if available.

  To define one or more decision variable cells:

  1. Select a cell or range of cells.

    Select value cells or blank cells only. You cannot define a decision on a formula or non-numeric cell.

  2. Select Define, and then Define Decision, Define Decision button.

    The Define Decision Variable dialog opens .

  3. Click the More button, More Button, to display all settings.

  4. Complete the Define Decision Variable dialog:

    • Name is the name of the decision variable.

    • Bounds are the upper and lower limits for the decision variable range.

    • Type defines whether the variable type is:

      • Continuous — can assume any value between the lower and upper bounds

      • Discrete — can assume values at specific intervals between the lower and upper bounds

        With Discrete selected, Step specifies the interval between values for discrete variables. For example Step = 1 could be used to specify whole dollars, while Step = .5 could specify 50-cent increments.

      • Binary — is 0 or 1 to represent a yes-no decision, where 0 = no and 1 = yes

      • Category — can assume any discrete integer between the lower and upper bounds (inclusive), where the order of the values does not matter. This type, used for attributes or indexes, is mostly used when numeric values represent conditions or groups instead of numeric values. Example model Groundwater Cleanup.xls contains a decision variable named Remediation Method, expressed as integers 1, 2, and 3. These are not numeric values, but instead they represent three different remediation methods for groundwater cleanup and can be defined using the Category type.

      • Custom — can assume any value from a list of two or more specific values or a cell range reference. When values are entered directly, separate them by a valid list separator — a comma, semicolon, or other value specified in the Windows regional and language settings. If a cell range reference is used, it must include more than one cell so there will be two or more values. Blanks and non-numeric values in the range are ignored.

        You can use cell referencing to name a decision variable, define the lower and upper limits, set the step size, and define custom values ( Entering Cell References and Formulas).

  5. Click OK.

  6. Repeat these steps for each decision variable in the model.

    Note:

    There is no absolute limit to the number of data cells you can define per worksheet. In general, you should define less than 1000 assumptions, decision variables, and forecasts per worksheet.