经济预测是一项"不可能的艺术"，叫做"现测"或许更恰当。最近经济形势的几次大逆转应该是个提醒：经济预测更善于指导对当前形势的感知，而非未来的结果。公众应该学会喜欢不确定性。
过去10天，经济合作与发展组织(OECD)和欧盟委员会(European Commission)都发布了最新经济预测。德国2.2%的惊人增长和英国1.2%的喜人增长，推动欧洲第二季度实现强劲增长。在这种形势下，和所有人一样，这两个机构也是手忙脚乱。但二者吸取了截然相反的教训。经合组织调低了秋季预期，大概是认为经济动力已经用完。欧盟委员会则预计，"一些增长势头……还将继续显现"，并调高了预期。
结果，两者的预期大相径庭，而事实很快就会证明，两家权威机构中至少有一家是相当错误的。对于欧元区核心国家，两者的分歧尤其引人注目，特别是由于我们已经进入了预测期。第三季度，经合组织认为法国和德国的经济产出仅能增长0.2%，而意大利会轻微萎缩。欧盟委员会更为乐观的预期则认为，三国都会增长0.5%左右。
如果就此认为经济预测就像是从帽子里随意抽取几个数字，未免有些无礼。此外，两家机构有一项预测是相同的：在大部分地区，2010年下半年的经济增长会慢于上半年。这一预测得到了大量经济指标的支持，如消费者信心指数和采购经理指数停止增长或下滑。难怪二次探底再次成为众人议论的话题。
不过，即使经济正在放缓，那也不是二次探底的征兆，而是经济复苏征途漫漫的印证。这是每次金融危机后的历史常态，本次危机最可能的结局也是如此。需求由于私营部门去杠杆化而受到抑制，很快还有公共部门的去杠杆化。至于供给方面，繁荣创伤最重的国家（如今失业的劳动力以前大多在建筑业工作）需要时间，才能将资源转移到更有价值、更可持续的活动中。而让总供给和总需求能够更轻松地彼此促进的信贷体系已经破损。
既然对未来的指导如此不可靠，人们该如何做决定呢？听从丹麦格言家皮亚特•海恩(Piet Hein)的建议也未尝不可——抛硬币。并不是凭运气做决定，而是因为"把一分钱抛到空中后，就会突然明白自己希望的是什么"。毫无疑问，我们希望的是，最后事实能够证明，欧盟委员会和经合组织所做预测中相同的那部分是错的。
Economic forecasting, that art of the impossible, is more aptly called nowcasting. As recent about-turns should remind us, economic predictions are better guides to current perceptions than to future outcomes. The public is advised to learn to love uncertainty.
In the past 10 days, both the Organisation for Economic Co-operation and Development and the European Commission have issued updated economic forecasts. Like everybody else, they were wrong-footed by Europe's strong growth in the second quarter, powered by Germany's roaring 2.2 per cent quarterly rate and a respectable 1.2 per cent rate in the UK. Yet they drew quite opposite lessons. The OECD lowered its predictions for the autumn, presumably thinking the economic fuel had run out. In contrast, Brussels expects "some momentum . . . to feed through" and nudged its expectations up.
The result are predictions so wide apart that at least one of the two august institutions will soon be proved quite mistaken. The divergence is particularly arresting for the core euro countries – especially as we are well into the forecast period. In the third quarter, the OECD thinks France and Germany are adding only 0.2 per cent to output – and it expects Italy to shrink slightly. The Commission's rosier forecast has all three press ahead by around half a percent.
It would be rude to suggest that this makes forecasts look like so many draws of numbers from a hat. Besides, the two bodies have one prediction in common: in most places output will grow less fast in the second half of 2010 than in the first. This is supported by a host of indicators, such as stalling or falling confidence measures and purchasing managers' indices. No wonder talk of a double-dip recession is again on everyone's lips.
Even if a slowdown is happening, however, it is less a portent of a double dip than confirmation that the recovery will be a long slog. That is the historical norm after financial crises; it was always the most likely dénouement of this one. Demand is restrained by private and, soon, public deleveraging. As for supply, it will take time for the most boom-scarred countries – where much now-unemployed labour worked in construction – to shift resources into more worthwhile and sustainable activities. And the transmission of credit, by which aggregate demand and supply could more easily bootstrap each other up, is broken.
With such unreliable guides to the future, how is one make up one's mind? One could do worse than take the advice of Piet Hein, the Danish aphorist. Flip a coin – not to leave choices to chance, but because "the moment the penny is up in the air, you suddenly know what you're hoping". Hoping, no doubt, that where their predictions agree, the Commission and the OECD will both be proved wrong.
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