hi, Peter
Likely best done in SQL. Such as create the pivot table in a file separate from the data & at the first step of the pivot table wizard (ALT-D-P) choose the external data option. Follow the wizard to the end - there will be some minor hurdles if the data doesn't have a normal defined name: can be overcome via PT wizard options, system tables - and then take the option to edit in MS Query. Then the SQL button and edit the SQL to suit. Via the 'open door' icon exit MS Query & complete the pivot table.
SQL will use IIF such as
SELECT IIF(value>0,'positive','not positive) AS [FieldName]
Google will find examples - I've posted many in the past. HTH. Regards