


This extension includes two versions of a Probability Distribution Calculator, each of which calculate distribution data based on a variety of distributions and parameters. The Probability Distribution Calculator is started from within a View, and is opened by clicking on the button in the View toolbar. You simply enter the input and parameter values, specify whether you are calculating Probability, Cumulative Probability or Quantile values, and click “Calculate”, and the result appears in the “Output Value” window. This calculator stays open until you close it and you can leave it open as you do other things in ArcView. The “Table Probability Distribution Calculator” is designed to work on all selected records in a table, applying the distribution parameters to each value and saving the results to a field in that table. This calculator is opened from within a Table by clicking on the button in the Table toolbar. Select the field containing the “Input” values, then decide whether to create a new field or use an existing field to save the “Output" values, then click “Calculate” to generate distribution values for all selected records.The window stays open until you click “Calculate” or “Cancel”. The Distribution functions included with this extension may be grouped in 3 categories. In general, the Probability Density Functions return the probability that the Test Value = X, given that particular distribution. The Cumulative Distribution Functions return the probability that the Test Value <= X, given that particular distribution. The Quantile Functions (sometimes referred to as Inverse Density Functions or Percent Point Functions) return the Value X at which P(X) = [specified cumulative probability], given that particular distribution.
Equations for each function are included in the Discussions of Distribution Functions, Parameters and Usages, but some them do not have closed formulas that can be calculated and therefore must be computed numerically. The author refers interested persons to the references to find source code and computational methods of calculating these functions. The author especially recommends Croarkin & Tobias (date unknown) and McLaughlin (2001) for illustrations of the various distributions, and Press et al. (19881997) and Burkardt (2001) for computational methods. These sources are all available online. The Discussions of Distribution Functions describe 4 methods of utilizing each function. The first method describes how to use the Probability Distribution Calculators to calculate values, and there are three additional methods available for programmers who want to access the functions through Avenue code. Simply copy the line of code exactly as written, substituting your parameter variable names in the proper places. Avenue Functions:
Discussion of Distribution Functions: Calculating Summary Statistics with Avenue Back to Statistics/Distributions  Summary Statistics  References Download Statistics Extension Manual (Adobe PDF) Please visit Jenness Enterprises ArcView Extensions site for more ArcView Extensions and other software by the author. We also offer customized ArcViewbased GIS consultation services to help you meet your specific data analysis and application development needs.
