Most of the fiscal impact models being developed today use spreadsheets to solve hundreds of equations. These complex spreadsheet structures, are difficult to understand and have a very high susceptibility to accidental errors introduced by mistyping, and do not have any kind of data validity procedures to maintain the integrity of the model.

System Dynamics a computer based simulation modeling technique developed at MIT in the 1950s by Jay W. Forrester provides a suitable alternative. System dynamics models use time as a variable and simulate the process over a period. These models can be used to assess the response of a system over a period of time. If we assume the process of comprehensive planning as one system composed of different elements, we can track or simulate its changes in response over a period of time using system dynamics methodology.

FIELD model was developed using, Powersim Studio 2005, one of the most popular tools available for System Dynamics. Unlike spreadsheets, Powersim has features like arrays that can be used to guide the interaction among the different components of the model using a single equation. This tool automatically checks the consistency of the entire model before running the simulation. The program gives a warning if there are undefined variables or inconsistent relationships.

Powersim offers excellent visualization capabilities using a range of tools like graphs, tables and gauges to show the results. The entire model is interactive and allows a user to change the key variables/assumptions and analyse the impacts shown by the model. We found Powersim and the System Dynamics methodology of immense help while working on the FIELD model, and would strongly encourage its use in the field of fiscal impact analysis.