PivotTable - Calculated Fields based on field of a previous week?

I have a PivotTable talbe with week number across the top and item down the side. Is it possible, with a calculated field, to lookup to a value from a previous week (column)?
For example, the calculated field for an item for week 2 is based on week 1 sales + week 1 forecasted sales.
I hope you can understand what I am asking for and that someone can help.

Originally Posted by stuartmarsh

I have a PivotTable talbe with week number across the top and item down the side. Is it possible, with a calculated field, to lookup to a value from a previous week (column)?
For example, the calculated field for an item for week 2 is based on week 1 sales + week 1 forecasted sales.
I hope you can understand what I am asking for and that someone can help.

I dont think so you can if you have another column in source data which you could refer to in the calculated field