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The increasing use of geographic information system (GIS) technology in various fields suggests the need for professionals, including those in Extension, to be mindful of communicating data accurately and effectively. This article examines approaches to creating classes or groupings within data as well as the weaknesses of each approach. Data break units discussed in the article include equal intervals, quantiles, and units resulting from natural breaks. Ideal situations for each type of data break are presented. The article emphasizes the need for Extension professionals to consider the effects of data grouping to avoid miscommunication when using GIS mapping. [To view "Part 1--Choosing Unit of Representation," see EJ1215045.]

Extension professionals are increasingly using geographic information system (GIS) technology to develop and inform programs and services. In this article, we use a mapping exercise to demonstrate how the unit of data can be applied and inadvertently misrepresented in GIS mapping. We contrast the use of counts, percentages, and location quotients with the same data and the resulting divergence in maps. The discussion addresses ideal circumstances for using each unit of data. Overall, the article illustrates the need for Extension professionals to be cognizant of the benefits and limitations of various units of data to avoid miscommunication when using GIS mapping. [For "Part 2--Determining Data Cutoffs," see EJ1215050.]