Using Formula Fields to Create Percent fields for Themes

Many users might want to show percentages of populations. This could be the percentage of a certain race, the percentage of Households that make a particular income range, or the percentage of home built in a particular decade. The formulas for each calculation will be different, but in all cases you will have to do three things: create a formula field(s), change the field properties, and apply the color theme. Please follow the steps below for each item.

Create a formula field (See this video for an overview)

1. Open a dataview for the layer (Census Tract, Block, etc.) you want to work with. You can do this by right-clicking the layer name and choosing “New Dataview”.
2. With the dataview open go to Dataview>Formula Fields and build a formula that calculates the value you want. If you want the percentage of the black population your formula would look like this: Black / Population. Enter this into the Formula by typing or using the Field List and Operator List under the Formula builder. It is important to use the correct column headings in this case so we would recommend using the Field list dropdowns.
3. Under the Formula Fields heading you will also need to name the field. Just click into the dropdown and type what you want the field to be titled, Percent Black, for instance.
4. Click OK to create the field and calculate the values.
5. This will add a new field to the dataview and should zoom the dataview to this field.

Change the field properties

1. Right-click anywhere on the dataview and choose “Field Properties”, or go to Dataview>Fields>Properties to open the Field Properties dialog.
2. Find the field heading for the field you just created (Percent Black, e.g.) and click in the “Format” area for that field.
3. Choose one of the percentage formats depending on the level of precision you want to show.
4. Click OK to close the field properties dialog.

Apply the color theme (See this video for an overview of thematic mapping)

1. Click back on your map or go to Windows > (Your map name) to go back to the map view.
2. Make the layer you are working with the working layer (right-click and choose “Make working layer”).
3. Click on the Color Theme button or go to Map>Thematic Mapping>Color
4. In the Color Theme dialog choose the field you created above and the classification method. The Styles tab allows you to set the colors that will be applied.
5. Click OK.

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