A histogram is a display of statistical information that uses rectangles to show the frequency of data items in successive numerical intervals of equal size. In the most common form of histogram, the independent variable is plotted along the horizontal axis and the dependent variable is plotted along the vertical axis. The data appears as colored or shaded rectangles of variable area.
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The illustration, below, is a histogram showing the results of a final exam given to a hypothetical class of students. Each score range is denoted by a bar of a certain color. If this histogram were compared with those of classes from other years that received the same test from the same professor, conclusions might be drawn about intelligence changes among students over the years. Conclusions might also be drawn concerning the improvement or decline of the professor's teaching ability with the passage of time. If this histogram 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 of the professors.
Some histograms are presented with the independent variable along the vertical axis and the dependent variable along the horizontal axis. That format is less common than the one shown here.
Compare pie graph.