Time phased rows for the same item. Build Expression based on data from different row

[neosilitone]

Hello,

NOTE: This is my first time posting on this forum so please excuse my ignorance.

Objective: Build expressions/calculations based on data from a prior row for the same key field/item.
Below/attached is an example of what I'm trying to accomplish and the logic used:


NOTE: INVENTORY is the current inventory populated for all weeks



Please let me know if you need further clarification.

Thanks!

[June7]

Here is one method http://allenbrowne.com/subquery-01.html#AnotherRecord