When the number of items of same type of industrial property is quite large, calculating depreciation for a group of such items may be more efficient than depreciating each item separately. Several different depreciation systems may be used for group depreciation. If the life of each vintage in an account are not estimated, then the BG procedure can be used; the BG procedure puts all vintages of the same type of property into a single broad group for depreciation purposes. In this case, only an estimate of the PASL and the net salvage ratio for all the property in the broad group is needed to calculate the depreciation charge