Composite or group depreciation is a depreciation system whereby

1. the years of useful life of the various assets in the group are added together and the total divided by the number of items.

2. the original cost of all items in a given group or class of assets is retained in the asset account and the cost of replacements is charged to expense when they are acquired.

3. the cost of individual units within an asset group is charged to expense in the year a unit is retired from service.

4. a straight-line rate is computed by dividing the total of the annual depreciation expense for all assets in the group by the total cost of the assets.