Economics notes: Discounting
Economics notes: Discounting
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经济学笔记：贴现
DOI：
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发表时间：
1999
期刊：
影响因子：
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通讯作者：
J. Raftery
中科院分区：
文献类型：
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作者：
D. Torgerson;J. Raftery
直到最近，在经济评估中，“贴现”未来成本和收益一直是常见的做法，但最近贴现收益已成为有争议的问题。贴现使当前的成本和收益比未来发生的成本和收益更有价值，因为现在花钱有机会成本，人们希望现在而不是将来享受收益。当前支出相对于延迟支出会产生机会成本的原因是，货币投资会产生真实的回报率，因此当前支出是有成本的。
例如，如果投资100英镑，名义回报率为10%，一年后它将价值110英镑;如果通货膨胀率为4%，这将导致每投资100英镑的真实的回报率为6英镑。如果出于某种原因，100英镑的医疗支出被推迟一年，那么（假设谨慎投资）我们可以预计，在一年的时间里，我们将有106英镑的医疗投资。
考虑到现在投资而不是等待一年的机会成本，我们必须贴现未来成本。因此，如果两项医疗保健干预措施都节省了100英镑，但其中一项我们必须等待一年，那么，在其他条件相同的情况下，我们将采取现在节省100英镑的干预措施。这是因为现在释放的100英镑，如果投资，将在一年内产生额外的6英镑（贴现率为6%）。
在经济评估中不对未来成本进行贴现会产生误导性的结果。例如，囊性纤维化筛查的评估显示，检测和终止一次受影响的妊娠的成本为80 000英镑。1将该成本与未来治疗囊性纤维化个体的额外成本进行比较，估计25年内每年为5000英镑。由于囊性纤维化筛查的益处（125，000英镑）超过了成本（80，000英镑），因此得出结论，筛查具有良好的性价比。然而，如果避免的成本被贴现（6%），那么这些成本将只有63917英镑，这改变了研究结果（尽管如果贴现率只有4%，就不会改变）。
贴现未来成本是没有争议的，直到最近，贴现与健康有关的福利的过程也是如此。反对将健康福利贴现的主要论点是，健康与财富不同，不能投资以产生未来收益。2因此，卫生署建议不应将与健康有关的福利贴现，3，4尽管最近的建议建议认为未来的健康福利应贴现，但贴现率非常低，为1.5%-2%。
不考虑未来成本和收益的一个重要原因是“时间偏好”，这是指希望享受现在的收益，同时推迟这样做的任何负面影响。人类行为隐含地低估了未来健康影响的例子比比皆是。例如，吸烟和饮酒给当前的快乐，而招致未来（折扣）有害的健康影响。事实上，研究表明，吸烟者对未来健康利益的重视程度低于不吸烟者。6这种在当前享受愉悦利益的愿望往往反映在商品和服务的差别定价上。考虑租用一个视频供家庭观看。尽管新发布的视频成本增加，但许多人愿意支付额外的成本，而不是等到价格福尔斯下降。
不贴现未来与健康有关的福利往往比贴现显示出更有利的成本效益比。例如，对乳房筛查的双视图乳房X光检查的评估显示，每生命年的未贴现边际成本为1200.7英镑。然而，贴现生命年（6%）使每生命年的边际成本增加了74%，达到2092英镑。
如果未来的健康福利不打折扣，这意味着今年取得的健康成果与20年或30年后取得的成果具有同等价值。举个例子，让我们假设100名妇女的髋部骨折预防费用约为7万英镑，考虑的策略有两种：10年的激素替代疗法（给50岁的女性），在30年内预防50%的骨折;或者10年的钙和维生素D（给70岁的妇女），这可以在10年内预防30%的髋部骨折。
在表中，我们显示了贴现健康效益如何改变两种干预措施的相对成本效益，以防止髋部骨折。如果不打折扣，激素替代疗法比维生素D和钙更能避免髋骨骨折。另一方面，如果髋部骨折的好处被打折，那么反过来也是正确的。
在这个例子中，关于采取哪种预防策略的决定受到折扣的严重影响。我们是否对健康福利打折，以及打折的幅度如何，取决于社会对当前健康福利与未来福利相比的重视程度。直觉上，在未来10年内预防髋部骨折似乎是最好的，而不是等待30年才能获得这种健康益处。关于社会对当前健康福利与未来健康福利的偏好的研究很少，这表明人们对当前健康福利的重视程度高于未来健康福利，而且人们实际上对未来健康收益的贴现率高于未来财富收益。6许多经济学家仍然认为未来健康收益应该贴现。9在大多数经济评估中，贴现率的选择不会影响贴现率。所评价的干预措施的相对排名。然而，通过使用不同贴现率的敏感性分析来确定贴现率是否对评价结果产生重大影响是一种良好做法。
Until recently it has been common practice in economic evaluations to “discount” both future costs and benefits, but recently discounting benefits has become controversial. Discounting makes current costs and benefits worth more than those occurring in the future because there is an opportunity cost to spending money now and there is desire to enjoy benefits now rather than in the future. The reason why current spending incurs an opportunity cost relative to delayed spending is that a monetary investment yields a real rate of return and therefore there is a cost to spending money in the present.
For example, if £100 were invested with a nominal return of 10%, in one year’s time it would be worth £110; if inflation was 4% this would result in a real return of £6 on every £100 invested. If for some reason £100 of healthcare spending were delayed for one year then (assuming prudent investment) we could expect that in one year’s time we would have £106 for healthcare investment.
To take into account the opportunity cost of investing now rather than waiting one year we have to discount future costs. Therefore, if two healthcare interventions both released £100 in savings but for one we had to wait a year, then, all other things being equal, we would adopt the intervention that saved £100 now. This is because the £100 released now, if invested, would produce an extra £6 in a year’s time (with a discount rate of 6%).
Failure to discount the future costs in economic evaluations can give misleading results. For example, an evaluation of cystic fibrosis screening revealed a cost of £80 000 for detecting and terminating one affected pregnancy.1 This cost was compared with the future excess costs of treating an individual with cystic fibrosis, which was estimated to be £5000 a year over 25 years. As cystic fibrosis screening benefits (£125 000) outweighed the costs (£80 000) it was concluded that screening represented good value for money. However, if the averted costs had been discounted (at 6%) then these would have been only £63 917, which alters the study’s results (though not if the discount rate were only 4%).
Discounting future costs is uncontroversial and until recently so was the process of discounting health related benefits. The main argument against discounting health benefits is that health, unlike wealth, cannot be invested to produce future gains.2 The Department of Health has thus recommended that health related benefits should not be discounted,3,4 though more recent advice suggests future health benefits should be discounted but at a very low rate of 1.5%-2%.5
An important reason for discounting future costs and benefits is “time preference,” which refers to the desire to enjoy benefits in the present while deferring any negative effects of doing so. Examples of human behaviour which implicitly discount future health effects abound. For instance smoking and drinking give current pleasure while incurring future (discounted) detrimental health effects. Indeed, research has indicated that smokers value future health benefits at a lower rate than non-smokers.6 This desire to enjoy pleasurable benefits in the present time is often reflected in differential pricing of goods and services. Consider the hire of a video for home viewing. Despite the increased cost of newly released videos, many people are willing to pay the extra cost rather than wait until the price falls.
Failure to discount future health related benefits will tend to show more favourable cost effectiveness ratios compared with discounting. For instance, an evaluation of two view mammography for breast screening showed an undiscounted marginal cost per life year of £1200.7 However, discounting the life years (at 6%) increased the marginal cost per life year by 74%, to £2092.
If future health benefits are not discounted this implies that health gains achieved this year and those achieved in 20 or 30 years are of equal value. As an example, let us assume that about £70 000 is available for hip fracture prevention in 100 women, and there are two strategies under consideration: 10 years of hormone replacement therapy (given to 50 year old women), which prevents 50% of fractures in 30 years’ time; or 10 years of calcium and vitamin D (given to 70 year old women), which prevents 30% of hip fractures in 10 years’ time.
In the table we show how discounting health benefits alters the relative cost effectiveness of the two interventions to prevent hip fracture. Without discounting, hormone replacement therapy produces a lower cost per avoided hip fracture than vitamin D and calcium. On the other hand if the hip fracture benefits are discounted, then the reverse is true.
In this example the decision on which preventive strategy to adopt is heavily influenced by discounting. Whether we discount health benefits, and at what rate, depends on how much value society places on current health benefits compared with future benefits. Intuitively it would seem best to be able to prevent hip fractures in the next 10 years rather than wait 30 years for this health benefit. What little research there has been into society’s preferences about current health benefits compared with future ones suggests that people value current health benefits more highly than future ones and that people actually discount future health gains more highly than future wealth gains.6 Many economists still hold the view that future health gains should be discounted.9 In most economic evaluations the choice of discount rate will not affect the relative ranking of the interventions under evaluation. However, it is good practice to establish whether the evaluation results are critically affected by the discount rate by a sensitivity analysis using different discount rates.