Help with selecting rows to do a calculation

[Adam_Dooley]

I have an odd one here that I can't figure out.  The data set is ordered sequentially by the first "item_number" field.  For most rows the "Amount" field alternates between a negative then positive value.  So then it is easy to summarize the values by the "Product Group" values to get the total.

However what I need to break out is the last 3 rows in yellow where the negative amounts are associated with one product group, then the corresponding positive amount is associated with a different group.  I need to identify all 3 rows in the dataset to then do further calculations downstream.  Unfortunately there isn't another field that would signify rows that *should* be related, so I have to come up with the rule.  

I believe a multi-row formula tool is required here, but can't figure out how to write the expression.  Any thoughts?

[T_Willins · Accepted solution]

Hi @Adam_Dooley,

It looks like you need a new grouping.  You are correct that a multi-row formula can do this.  Hopefully this helps!

[Adam_Dooley]

That gets me what I need, thank you @T_Willins !!