When specifying fields at the Data tab, click 
to add a field that is calculated from other data rather than selecting one from the list of available fields.
•Title is displayed on the report or chart and used to reference the field.
•Expression determines the calculation that is undertaken, which is not input directly but is created by clicking 
to open the Expression Builder.
•Evaluate Null as 0 indicates, if checked, that an expression that fails to deliver a valid value will have the value set to zero.
•Type determines how the result of the calculation is to be presented; a type of Decimal will require the input of the number of decimal places.
Indicate how the resulting value is to be shown in sub-totals and totals on change of the fields specified under Group Rows by (selections will vary depending on the specified Type):
o Sum — total of the accumulated field values
o Min/Max — the lowest or highest value within the group
o Expression — use the calculated field expression against summarized values to derive a total
o Count — the number of records since the last change of group value
o Average — the total of accumulated field values divided by the number of rows
o Unique Count — the number of records since the last change of group value, ignoring those with duplicated field content (such as the same name).
Check Cumulative if the relevant value is to be accumulated through subsequent sub-totals for a grand total or is to be cleared down to zero on each change of group value.
o Heading Group is a selected or entered name that is to appear at the head of several grouped columns with the same heading group name (such as grouping several types of cost).