Click New and select Column.
Click to select a value column in the Data Sources tree, for example a Sales column.
Click the topmost Add > button.
In the Expression field, type:
AVG(%1)
Comment: AVG is the average function. You can find more aggregate functions here.
Click 
next to Group By to show the controls determining over which column the aggregation will be made.
Click New to add an empty group by expression.
Click to select the column to group by in the Data Sources tree, for example a Type column.
Click Add > in the Group By Settings section.
Click Save.
Response: The resulting column will consist of the average of the value column, for each value in the group by column. Using the example columns above it would be a column containing the average sales for each type of product.