The reasoning behind both adjustments is much the same. A portion of capital investment each year is used to replace depreciated capital. Deducting depreciation from either the total production of all goods (GDP) or just the production of capital goods (GPDI), leaves the "net" addition to production that can be used to move the economy forward.

Three Sources of Depreciation

• First, capital simply wears out through use. As the wheels of industry roll on, metal grinds against metal, wood becomes weathered, and the forces of nature take their toll. The tread on a delivery truck tires wears down. The cutting edge of a carpenter's saw wears off. The hard drive in an accountant's computer wears out. The fact of life is that capital wears out. It happens.


• Second, capital breaks down through constant use, accidents, or natural disasters. Today the capital works fine, tomorrow it does not. A carpenter's saw breaks when it encounters an undetected nail in a two-by-four. The tire of an delivery truck blows out when the tread is thin and the pavement is rough. An accountant's computer malfunctions after being tossed from a thirty-seventh-floor window (something about an undecipherable change in the IRS tax code).


• Third, capital becomes less productive through technological obsolescence. As technology marches forward, capital becomes less valuable and less productive even though it is in perfect physical condition. An accountant's computer might function flawlessly, but it lacks the memory and speed to run the latest software. A delivery truck works okay, but it just is not as big enough or as fast enough to make the needed deliveries. A carpenter's saw still works, but the newer ones with tungsten steel blades cut the wood even better. In some cases, technology advances make it more expensive to operate old capital than to purchase new capital.

Currency Depreciation