Computer models hold a continuing fascination for political scientists. Part of their attraction derives from being working miniatures like mechanical toys. They also hold the less enjoyable but more scientific promise of allowing something akin to laboratory experimentation—denied to political scientists in most circumstances because of cost or the nature of the subject matter.
Spreadsheets are powerful tools for the construction of simple or complex models. At their core, spreadsheets are nothing more than cells arranged in columns and rows into which labels, numbers, or formulas can be placed. The numbers and formulas can be arranged in infinite variety, and in modern spreadsheets they are supplemented by such features as built-in formulas called functions, macro languages which are highly flexible programming environments, data query features, and graphics capabilities.
Edith Stokey's and Richard Zeckhauser's clearly explained difference equation models lend themselves especially well to rendition on a spreadsheet (1978, 47–73). Stokey and Zeckhauser furnish the case of public housing units in a city deteriorating at the rate of 10% per year to the point of being uninhabitable. If 800 units per year are built, will the total number of public housing units reach an equilibrium and, if so, will it be stable?
Such a problem is expressed in difference equation form as follows:
Unitsn = Unitsn − 1 – .1
* Unitsn − 1 + 800
Unitsn = .9 * Unitsn − 1 + 800
The number of housing units in any year n equals .9 times the number of housing units in the previous year plus 800. The beginning of a spreadsheet solution to this problem is shown in Figure 1.