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Definition

pie graph (or pie chart)

Compare with histogram.

A pie graph (or pie chart) is a specialized graph used in statistics. The independent variable is plotted around a circle in either a clockwise direction or a counterclockwise direction.The dependent variable (usually a percentage) is rendered as an arc whose measure is proportional to the magnitude of the quantity.Each arc is depicted by constructing radial lines from its ends to the center of the circle, creating a wedge-shaped "slice."The independent variable can attain a finite number of discrete values (for example, five).The dependent variable can attain any value from zero to 100 percent.

The illustration is a pie graph depicting the results of a final exam given to a hypothetical class of students.Each grade is denoted by a "slice."The total of the percentages is equal to 100 (this is important; if it were not, the accuracy of the graph would be suspect).The total of the arc measures is equal to 360 degrees.

pie_graph.gif  (2552 bytes)

From this graph, one might gather that the professor for this course was not especially lenient nor severe.It is evident that grading was not done on a "pure curve" (in which case all the arcs would have equal measures of 72 degrees, corresponding to 20%).If this graph were compared with those of classes from other years that received the same test from the same professor, some conclusions might be drawn about intelligence changes among students over the years.If this graph were compared with those of other classes in the same semester who had received the same final exam but who had taken the course from different professors, one might draw conclusions about the relative competence and/or grading whims of the professors.

This was last updated in March 2011
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