Equal interval classification

Equal interval (sometimes called Equal steps) classification. In this method the class width is equal for all classes. The technique works by subtracting the highest value from the lowest value, and the result is divided by the number of classes. Let us assume we want to create 5 classes out of this data set: ((1000-10)/5)= 198. This result (198) is used as a constant C in the following formula to determine the class boundaries:

Lowest value + C + C + C + C + C = highest value

Equal interval classification is appropriate when the data values are reasonably evenly spread throughout the data range. Examine the histogram: if the histogram has a fairly flat trend, the data values are evenly spread.

Choropleth map using Equal steps classification method

Fig 1. Demographic map, six classes. Equal interval classification ; monochromatic greyscale.

Exercise: Using the interactive classification applet determine a variable suitable for classification using the Equal interval method and map it.

You can open the applet in a new window by clicking on this link. and refer back and forth with this window when needed.

  1. Read the applet instructions and familiarise yourself with the applet features.
  2. Load the Perth data. This is explained in the instructions.
  3. Select a variable from the variable list. Remember derived data is suitable and total data is not suitable.
  4. Examine the histogram for the data.
    • To do this you will need to click the <Explore> button followed by the <Diagram> button followed by the <AlphSort> button.
    • You will be presented with a histogram of the data values for each enumeration district ( post code).
  5. You will seek variables that show relatively even spread of the data values¼or at least no dramatic trends!
  6. Once you have found suitable data you can map it using the Equal interval classification method. Experiment with a different number of classes and interpret you results.

Hint: Try mapping Percent born in UK & Ireland and also Population born in Vietnam per Km2.