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5.2 Pie charts

"Which graphic display techniques do we have for a site-typological analysis, based on the table of artefact data per site (Table 8, 4.1)?"

Exploratory Data Analysis provides a number of ways to display multivariate information graphically: the well-known Tukey stars, but also the so-called Chernov faces (Farley et al. 1990) are used. We feel, however, that the latter provides not enough structure and clear variation for a good display. Multivariate data may also be presented in pie charts or segment diagrams. A major drawback in pie charts is that a variable is not always in the same position in the circles. The percentage of macrolithic artefacts, for example, will not always be to the left in a pie chart, depending on the other percentages. That makes it difficult to compare various pie charts. Moreover, pie charts are used exclusively to express percentages, deemed less desirable in this investigation. Tukey stars and segment diagrams do not have these drawbacks. The position of a variable is set and both may be used to compare numbers, percentages, rankings or classes. In a Tukey star the value is expressed by the length of a line. A long line from the centre indicates the presence of large numbers of macrolithic artefacts. All ends are connected by a linking line. The star shape of that outer line is the graphic representation of a multivariate phenomenon. The visual impression of the shape of the Tukey star, however, is strongly determined by variables with small numbers. The perimeter line will then return to the centre and strongly influence the shape of the star. By using a linking line the variables are 'visually' connected. The emphasis on certain variables can be affected by changing the sequence of the variables. In a segment diagram the value is expressed by the size of the entire segment. Large numbers of macrolithic artefacts are represented by a pie wedge with a large radius. The segments are all of equal size and 'visually' independent of each other. Changes in the sequence of the variables leads to smaller changes in shape than in Tukey stars.

[Pie, Tukey and segment diagram]
Fig. 40 Pie chart, Tukey star and segment diagram for the artefact composition of the same site


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Last updated: Wed Feb 25 1998