Equal Interval classification

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Equal Interval classification

In Equal Interval Classification each class occupies an equal interval along the number line. They are found by determining the range of the data. The range is then divided by the number of classes, which gives the common difference. The class limits are established by starting at the lowest value and adding the common difference to get the upper limits of the first class, adding the common difference to this to get the limit of the second class, until the upper limit of the data is reached.[1]

Example: Equalintclass.PNG

Equal interval is useful when distribution of the data has a rectangular shape in the histogram. However, in geography, equal interval is most common when the classification units are nearly equal in size.

Advantages and Disadvantages

One advantage of using equal interval classification is that the steps to compute the intervals can easily be completed using a calculator or pencil and paper. A second advantage is that when the results of this classification are projected onto a map they are easily interpreted. Another advantage is that the legend limits contain no missing values or gaps. This permits faster map interpretation, but might create confusion concerning the bounds of each class.[2]

The main disadvantage of this classification type is that it fails to consider how data are distributed along the number line. For example, the map to the right shows the percentage of total homes in Arkansas which are mobile homes. There are many areas that fall into the two lower percentages, leaving most of the state the two shades of green. If a different classification was used, the data displayed in the map could be shown more effectively.

Equal Interval Map.PNG Equal Interval.PNG
Equal Interval Map Legend


  1. Tyner, Judith (2010). Principles of Map Design", page 167. The Guilford Press: New York, NY.
  2. Slocum, Terry A. (2009). "Thematic Cartography and Geovisualization", page 60. Pearson Prentice Hall: Upper Saddle River, NJ.



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Further Reading

See Also