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

1. In Edit Data Set, choose Computed Columns.
2. Choose New to create a new computed field.
3. On New Computed Column:

1. In Column Name, type the name of the computed field.
2. In Data Type, select a data type for the computed field.
3. In 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 expression builder button to open the expression builder. In Category, select Available Data sets, then select your data set.
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
2. Choose OK to save the expression. The expression appears in the Expression field in New Computed Column.

4. Choose OK to save the computed field. Computed Columns displays the computed field and the expression you defined.

4. To see all the columns that are specified in the data set, choose Output Columns. The computed field you created appears on this page.
5. Choose Preview Results to confirm that the computed field returns the correct data.
6. Choose OK. BIRT Report Designer saves the data set.