Which of the following is not a method of depreciation?

Accumulated cost depreciation is not recognized as a legitimate method of depreciation in accounting practices. Instead, it refers to the total amount of depreciation that has been applied to an asset over its useful life. It is essentially a cumulative figure representing the depreciation expense recorded to date, rather than a method used to systematically allocate the cost of an asset over time.

In contrast, the other options represent established methods of depreciation. Linear depreciation, also known as straight-line depreciation, evenly allocates an asset's cost over its useful life. Declining balance depreciation accelerates the depreciation expense in the earlier years of an asset's life, reflecting a larger expense initially and reducing amounts in subsequent years. The sum-of-the-years'-digits method is another accelerated depreciation technique that calculates depreciation based on the remaining life of the asset, favoring earlier years of use.

These recognized methods all serve to allocate the cost of an asset systematically but differ in how and when that cost is expensed. Accumulated depreciation, however, is merely a tracking mechanism rather than a method for calculating depreciation expenses.