Access to time-variant parameter specification (the Change Values form) can be from either guided scenario analysis or self-managed scenario analysis.

Although most modeling discussions portray parameters as if they should be fixed over time, that is a very limiting conceptualization of them. In fact, it is normally better to specify parameters so that a particular phenomenon (e.g., a change in values concerning fertility, a policy-influenced movement towards higher savings rates, or a development of renewable energy technologies) phases in over time.

Alternative Ways to Use the Change Values Form:

Use the Slider Bar to Change a Parameter for All Years. You move the slider to the left or right to change the parameter value and then touch the Register Change button to actually change the parameter.

Specify a Desired Value for One or More Years. You specify a desired numerical value, indicate the number of years you wish to repeat that value (one or more) and then touch the Change/Repeat button.

Interpolate to a Desired Value over Several Years. You specify a desired value, indicate the number of years over which you wish to interpolate to that value, and then touch the Interpolate button.

Move Forward or Backward Across Years. You touch the Previous Year or the Next Year buttons to move across time without changing parameter values.

Cancel all Changes. You touch the Cancel all Changes button to revert to the parameter values before you began making changes.

Example. Try increasing the value of agricultural yields (YL) in Mexico by raising the value of a parameter called "ylm" from 1.0 in the initial year to 1.3 in 2020. That would build in an assumption of a 30% increase in the productivity of African agriculture, relative to the base case. To do this, select Scenario Analysis from the Main Menu and the Self-Managed Scenario Analysis sub-option. On the Self-Managed Scenario Analysis form select the Change option and Full Set sub-option. Specify ylm and choose Mexico as the country/region. Success in doing that will take you automatically to the Change Values (time-variant parameter specification) form. Designate 1.3 as the desired value to which you will interpolate (that is, move gradually over time) and indicate the number of years for the interpolation (say 20). Select the interpolate action option to carry it out. Then identify 1.3 as the desired value you wish to repeat (that should already be done for you), 100 or some other large number as the years to repeat, and select the change/repeat button. Exit and select the Display option. Select ylm for display, and look at it in a table or graph to make sure you have changed this parameter as you desired. It is often a good idea to check the success of a parameter change before running the model.