#### How to add a computed field to a data set

1. In Edit Data Set, choose Computed Columns.
2. Under Column Name, select the first empty row, and type the name of the computed field.
3. Under Data Type, select a data type for the computed column.
4. Under Expression, specify the expression to calculate the desired value. You can either type the expression or use the expression builder to construct the expression. To use the expression builder, complete the following steps:

1. Choose the ellipsis (...) button to open the expression builder.
2. In Category, select Available Data sets, then select your data set.
3. In the top pane, type an expression. You can also double-click an item to add it to the expression at the insertion point. Figure 3-10 shows how to create an expression for a computed column.

Figure 3-10 Creating an expression for a computed column
4. Choose OK. Computed Columns displays the computed field and expression that you created. To see all the columns that are specified in the data set, choose Output Columns.

5. Choose Preview Results to confirm that the computed field returns the correct data.
6. Choose OK. BIRT Report Designer saves the data set.