Depreciation
Another source of problems is the measurement of depreciation, which is a key factor in computing earnings. The accounting and economic measures of depreciation can differ markedly.
According to the economic definition, depreciation is the amount the firm must reinvest to
sustain its real cash flow at the current level.
The accounting measurement is quite different. Accounting depreciation is the portion
of the original acquisition cost of an asset that is allocated to each period over an arbitrarily
specified life of the asset. This is the figure reported in financial statements.
The United States actually allows two sets of depreciation treatments. For tax reporting, it
currently allows the entire investment to be written off immediately, thus offering the firm a
tax break equal to the corporate tax rate, 21%, times the investment. However, for reporting
purposes, firms are more likely to depreciate an investment smoothly over some assumed
lifetime, for example, using straight-line depreciation.
Assume, for example, that a firm buys machines with a useful economic life of 20 years at
$100,000 apiece. In its financial statements, however, the firm depreciates the machines over
10 years using the straight-line method, for $10,000 per year in depreciation. Thus, after 10
years, a machine will be fully depreciated on the books, even though it remains a productive
asset that will not need replacement for another 10 years.
In computing accounting earnings, this firm will overestimate depreciation in the first 10
years of the machine’s economic life and underestimate it in the last 10 years. This will cause
reported earnings to be understated compared with economic earnings in the first 10 years
and overstated in the last 10 years.
Depreciation comparability problems add one more wrinkle. A firm can use different
depreciation methods for tax purposes than for other reporting purposes. Most firms use
accelerated depreciation methods for tax purposes and straight-line depreciation in published financial statements. There also are differences across firms in their estimates of the
depreciable life of plant, equipment, and other assets. Under current U.S. tax law, firms may
depreciate the full cost of a capital investment in the year it is put into use, but this is not the
case in most countries.
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Another complication arises from inflation. Because conventional depreciation is based on
the historical cost rather than on the current replacement cost of assets, measured depreciation
in periods of inflation is understated relative to replacement cost, and real economic income
(sustainable cash flow) is correspondingly overstated.
For example, suppose Generic Products, Inc., has a machine with a three-year useful life
that originally cost $3 million. Annual straight-line depreciation is $1 million, regardless of
what happens to the replacement cost of the machine. Suppose inflation in the first year turns
out to be 10%. Then the true annual depreciation expense is $1.1 million in current dollars,
while conventionally measured depreciation remains fixed at $1 million per year. Accounting
income therefore overstates real economic income.
概括
折旧是计算收益的关键因素，但会计折旧与经济折旧存在显著差异。经济折旧指企业为维持当前实际现金流所需再投资的金额，而会计折旧则是将资产原始成本按人为设定的使用年限分摊到各期间。美国允许两种折旧处理方式：税务申报时可全额抵税（享受21%税率优惠），但财务报告通常采用直线法逐年分摊。
例如，某公司购买使用寿命20年的机器（单价10万美元），却在财务报表中按10年直线法折旧（每年1万美元）。这导致前10年会计收益被低估（折旧高估），后10年则被高估（折旧低估）。不同企业可能采用不同折旧方法（税务加速折旧vs财务报表直线法）或资产寿命估计，且美国允许资本投资当年全额抵税，但多数国家不允许。
通胀进一步复杂化问题：传统折旧基于历史成本而非重置成本，通胀期间折旧被低估，实际经济收入（可持续现金流）则被高估。例如某机器原价300万美元（3年寿命），年折旧100万美元。若首年通胀10%，实际年折旧应为110万美元，但会计仍记录100万美元，导致会计收入虚增。
