I need to produce a report  that will calculate values based on previous record field value and current record field value

Can I store a field value say in a variable and then on the next detail line record have it accessed  so I can say subtract current field value - previous field value from previous detail line?

Where can I define this globial variable ?

Any help is really appreciated

Also I forgot to mention there is a next and previous function.  These might be useful for what you are doing.