An assumption is a value that is used as input to an analysis. They are usually changeable and they always apply to an entire scenario. Assumptions can be a way to express subjective inputs, such as how much weighting to give to a particular community value like open space or economic development. Output values that depend on a particular assumption are automatically updated when the assumption is changed and you click the Apply button.
An assumption can be designated as a numeric value within a valid range (as might be displayed in a slider bar) or as a choice (number, text, or Boolean – yes/no) associated with a defined set of valid values. A numeric assumption can be any number, rate, or standard (number of stories, gallons per household, cost per mile, % discount). “Choice” assumptions may represent a type (dirt, gravel, paved) or time frame (1990, 2000, 2010).
Assumptions can be referenced in any analysis formula. All analysis calculations that depend on an assumption value will be automatically recomputed if you modify that assumption value.
You can specify whether an assumption is fixed or variable.
A variable assumption is an input to the analysis that might change as part of the analysis, such as the current interest rate, seasonal resource consumption values, residential density, or survey results. A variable assumption may be altered during analysis using a slider bar, choice button, or drop-down list, and it can vary across scenarios. Slider bars are excellent for setting relative weighting factors. To show the relative weight of each of several factors, plot all their settings on one pie chart.
A fixed assumption is an input to the analysis that will not likely change, such as the municipal water supply. A fixed assumption value cannot be altered and has the same value across all scenarios.
For setup instructions, see Creating a new assumption.
For instructions on how to change assumption values during an analysis, see Changing assumption values.