Change Selected Functions

Variables are forecast based on mathematical relationships that are represented by functions within IFs. These functions can be changed based on different understandings of relationships between variables. Changing functions gives you a powerful tool for using IFs to investigate possible futures and, to an extent, to change the model itself. Change the relationships between two variables or multiple variables using the Bivariate and Multivariate function pages.

Bivariate Functions

To open the Bivariate Functions page from the Main Menu of IFs: choose Scenario Analysis, Change Selected Functions, and click Bivariate Function. Once on the Bivariate Function page, search for a particular function in the Filter Functions (press enter) field or scroll through the Function (click to see/edit) dropdown to view a relationship that is already in IFs. Use the filter options (Used in Run, Used in Historical Run, etc.) to the right of the dropdown to filter for functions that are used for particular model tasks, or not used at all.

An example of the Bivariate Function page used to change functions in IFs.

When a function is clicked, the relationship of the function will be shown in graph form at the bottom of the screen. Depending on the type of function, an R-squared and standard error of the relationship may also be shown below the graph. Export the graph by clicking on the three lines in the upper right corner of the graph, then select the desired format to export.

To change a chosen function, use the Table Functions Points frame option or the Analytic Function button option to specify a new relationship.

Table Functions Points: A table function relationship is based upon lines that connect given points. In the Table Function Points frame, there are boxes for specifying the X-axis and Y-axis values. Specify a desired point by entering the X and Y axis values, then click Add. Add as many points as desired, and click Previous and Next to move between already created points. Click Delete to remove a point from the graph. When the points desired to create a function with are all added, click Alter to alter the function to now be based on this new relationship.

Analytic Function: Click to open the Specify Analytic Function page. Use this page to change the mathematical formula of a function by adjusting its function terms. A function based on a linear regression will only have a and b1 terms, and altering other bs will change the whole function type. More complex functions will use different b terms, for example, an exponential will use b2 and a logarithmic b4.

An example of the Specify Analytic Function page, for GDP and car sales.

Change the Display Figure from X Value and To X Value fields to change the graph in the Bivariate Functions screen. Once the function is changed as desired, click Save and Continue to save and return to the Bivariate Functions page or Exit - Abandon Changes to return without saving the change.

Click Reverse Changes to return to the default for the selected function, Help to open the corresponding page in the Pardee Wiki, or Exit to return to the Main Menu. After clicking Exit, IFs gives a very important informational warning on how any function changes will be active only in the current session of IFs. Starting IFs again resets all functions to standard values. Moreover, although runs of the model that you make with altered functions will reflect the changes in functions, no information about the changed functions is saved with the model Run Files. Keep track of these changes somewhere for replicability.

Now run the model using the Quick Scenario Analysis with Tree option from the Main Menu. Once the model has run, a new working file is loaded that reflects the changed functions and their impact on all computations. Compare this working file (or a saved run file version of it) with the base case or with other scenarios.

Multivariate Functions

To open the Multivariate Functions page from the Main Menu of IFs: choose Scenario Analysis, Change Selected Functions, and click Multivariate Functions. This page displays a table with multivariate function names, their intercept, R-squared, and standard errors. Search for a particular function using the search bar or scroll through the table to find the desired function. Use the filter options (Used in Run, Used in Historical Run, etc.) to the right of the table to filter for functions that are used for particular model tasks, or not used at all.

An example of the Multivariate Functions page.

Click on any intercept to change the intercept term or on any function name to alter the other terms used in the function. A new page will open, where changes to function terms can be made.

An example of the Change Multivariate Function screen for Bilateral Aid Exports.

Click on the desired b coefficient to change the value for that coefficient. The b coefficients are based upon the standard function shown above the table; for example, b4 is for logarithmic functions. Once clicked, enter the desired value, then click Continue to save that term and repeat for each coefficient term desired to change. Then click Continue to save and update the function. Remember, starting IFs again resets all functions to standard values, and although runs of the model that you make with altered functions will reflect the changes in functions, no information about the changed functions is saved with the model Run Files. Keep track of these changes somewhere for replicability.

Again, run the model using the Quick Scenario Analysis with Tree option from the Main Menu to see the results of the altered functions. Once the model has run, a new working file is loaded that reflects the changed functions and their impact on all computations. Compare this working file (or a saved run file version of it) with the base case or with other scenarios.