Is there any way that i can add an extra column to a pivot table that I can
add a function into?

We have 2 internal quality checks for our product. I have created a log
which shows what type of issues are discovered in the first and second
quality check.
The pivot table in its most basic form shows the count of the different type
of issues found in the first QA check in the rows and the type of issues
found in the second QA check in the columns.

The nature of this layout means that looking at the Grand Total is
meaningless from a process analysis point of view. What I would need is to
add an extra column to the table which calculates the difference between the
items which were not the same on the first and second QA check (i.e. they
passed the first time, but failed the second time or vice versa).

Any help with this would be greatly appreciated as I am currently using the
cells outside of the table to make these calculations and that poses problems.