pure sector allocation

[kvjay]

The formula is as follow:
sum (Wpj - Wbj)(rbj - rb)
The explanation was that assumption- PM will held the same security in p/folio as in benchmark and in the same proportion as in benchmark
My Question:
If that is the case dont the first part of the formula will be always zero (Wpj - Wbj) ?? Pls help if I miss anything, thx

 

[sha_carsie]

If sector allocation is to determine the amount of alpha derived from the manager’s active decision to overweight or underweight (the sector), meeting the benchmark’s allocation would mean there was 0 value added from this decision.