Sumation of values for current period based on missing for prior

[sicilianif]

I have a need to be able to, dynamically, sum values for materials in a current period based on the fact that it is missing in a previous period. Below is an example of what I am talking about:

Plant, Material, Month, Usage
Essex, A, 3/2010, 10
Essex, A, 4/2010, 15
Essex, B, 4/2010, 12
Portland, C, 4/2010,10
Portland, D, 3/2010, 4
Portland, E, 3/2010, 5
Portland, E, 4/2010, 12

I would like to be able to show a summary level chart that would include, for each plant, the total usage for items in April, that were not used in March.

So based upon the data above, the chart would show :

Essex - 12 <--- Item B was not used in March, but was used in April
Portland - 10 <-- Only item C was not used in March, but in was used in April

I am able to make this work if I include material as one of the dimensions, but I can't figure out how to do it without the material.

I appretiate any and all help,

Frank

[#2 · Accepted solution]

I've made an assumption that for March you need to see products that was not used in February, in other words - All for March and Essex B & Portland C for April.

Take a look at attachment.

[sicilianif]

Great solution. The FieldIndex is the key. I think I will drop the prevkey and just do it on the fly by concatenating the key at run time. That should allow me to compare any month to any other month.

Thanks...