Hello
The report should be created in two steps. First, data should be summarized up to po level and later the average of all po should be calculated with drill down by material group. See example :
Life cycle
Order1 Material1 Material Group1 10
Order1 Material2 Material Group2 10
Order1 Material3 Material Group3 10
Order1 Material4 Material Group4 10
Order2 Material5 Material Group1 20
Order2 Material6 Material Group2 20
Overall aggregated average = (10+20)/2 = 15
Average per material group:
M1 = (10+20)/2 = 15
M2 = (10+20)/2 = 15
M3 = 10
M4 = 10
The po number should be out of report.