September 30, 2008
To facilitate the development of a Dirichlet distribution for a hierarchical partition of events and event groups, the Dirichlet Elicitation Tool (DET) was developed using Microsoft Excel as the main programming tool. This document, in draft form at this time, describes the various sections of the tool and also leads the interested reader through a “mini-tutorial” about how to use it. From a conceptual perspective, the tool assumes that it is used by an expert in the particular domain for which a Dirichlet distribution is to be constructed. It also assumes a “hierarchical” partition of the event space. By that, we mean that the user can specify a list of groups for which he or she then will specify individual events. In addition to labeling and describing these groups and events, the user’s main task is to specify split-ratios of the likelihood of the events within each group as well as split-ratios of the relative likelihoods among the most likely events from each group. The user also has to provide, in a very interactive manner, a scaling constant k that determines the shape of the marginal beta distributions. Based on these judgments the tool then calculates and reports the event probabilities for the resulting Dirichlet distribution. The programming mainly relies on built-in Excel formulas and functions that dynamically customize the tool and the required judgments depending on user input. A minimal amount of code written in Visual Basic for Applications (VBA is a component of Microsoft Excel) was required to navigate the tool. Since VBA is not part of the Macintosh version of Excel, the DET only runs under the Windows operating system. This draft document describes the individual sections of the DET and provides an example that can be used as a “mini-tutorial” for exemplary purposes only.