In some cases, you may not be sure what values to input for the data in your model. For example, suppose your model uses the inflation rate as a parameter. You may be uncertain as to the exact rate of inflation in the future, but you know it will most likely fall within a range of 2 to 6 percent. What you would like to do is run your model for various values of the inflation rate within this range to see how sensitive the model's results are to inflation. We refer to this as what if analysis, and LINGO has a feature to facilitate this. To set up a parameter for what if analysis, input a question mark (?) as its value in place of a number as in the following example:

DATA:

  INFLATION_RATE = ?;

ENDDATA

LINGO will prompt you for a value for the parameter INFLATION_RATE each time you solve the model. Under Windows, you will receive a dialog box resembling:

Simply input the desired value for the inflation rate and then press the OK button. LINGO will then set INFLATION_RATE to the value you input and proceed with solving the model.

On platforms other than Windows, LINGO will write a prompt to your screen asking you to input a value for INFLATION_RATE. Type in the value and then press the enter key.

In addition to parameters, you can perform what if analysis on individual members of attributes by initializing them to question marks in the data section, as well.

For an example of a model that uses what if analysis to compute the value of a home mortgage, see the WHATIF model in Additional Examples of LINGO Modeling.