Hi Rae Ann,
Given just a customer ID and invoice date and no other data preparation you'll need to use table calculations for this with formulas like LOOKUP(ATTR([Invoice Date]),-1) and LOOKUP(ATTR([Invoice Segment]),-1), respectively. The reason why is that comparison to prior is a task in Tableau that requires using table calculations.
I'm not sure why you wanted to use a FIXED LOD expression? If it was that you wanted to use the results of the lag calculations as dimension(s) then there are potentially workarounds using additional table calculations, sometimes those aren't feasible and we end up having to do data preparation outside of Tableau. If you can give me more details then I should be able to provide some guidance.
Thanks for your response! Here is a more detailed example that may better explain why I need this fixed at the customer ID. The goal is to flag any invoices that could be determined a "follow up" order.... meaning the customer bought something in the store and then had an online order within 2 days after.
For customer 2, if this were not fixed at the customer ID, then record 2 would indicate that this is a "follow up" order when in reality it is not, it is customer 1's order.
We can have our data team make us this field in SQL (since I know Tableau isn't necessarily a data manipulation tool), but the need to do a table calc (look up function) within a fixed calc comes up for me a lot so wanted to be sure there wasn't a work around for this in Tableau.