Economic fortune tellers let down by reality
President Harry Truman reportedly sought a one-armed economist, one who would be unable to say "on the one hand . . . and on the other". The joke is that there are more opinions than there are economists. But this is not true of economic forecasters. They all agree with each other. They all make the same mistakes.
In an article almost 10 years ago, I observed that between 1991 and 1994 Britain's growth rate had never once fallen within the range of forecasts made for it - everyone had overestimated growth in 1991 and 1992, and underestimated it in 1993 and 1994. Recently I decided to see whether matters have improved since then. The answers are not encouraging.
The UK Treasury publishes a compendium of economic forecasts. I looked at the 43 forecasts of growth in 1999 that it listed in December 1998 - the last moment at which people were still predicting the future. The average forecast was 0.8 per cent: 33 estimates were in the range 0.3 per cent to 1.3 per cent; and the complete range of forecasts extended from minus 0.5 per cent to 2.1 per cent. The actual growth rate in 1999 was 2.8 per cent.
For 2000, every single forecasting group again underestimated the actual growth rate. But the problem is not simply pessimism. Only two estimates for 2001 were too low.
The economic environment since then has been more stable but the degree of clustering has become still more extreme. In December 2003, for example, all but three forecasts of growth for 2004 were between 2 per cent and 3 per cent, and more than half were in the range 2.5 per cent to 2.9 per cent. This nearly universal agreement does not reflect common knowledge of what is actually going to happen. Six months later, there is similar unanimity that growth in 2004 will be above 3 per cent.
So, if you have one economic forecaster's opinion, you have them all. The basis of almost all forecasting is that the economy will revert from where it is now to its trend level. Whether people reach this conclusion by econometric modelling or putting a finger in the air, the answers they reach are similar. Forecasting accuracy has recently improved because the British economy has grown in line with that trend. In fact, if you had simply assumed since 1998 that the British economy would always grow at its long-term average of 2.5 per cent, you would have tracked the outcome more closely than if you had followed the consensus forecast.
But there is always a consensus forecast because the City pundits who form it watch each other and talk to each other. Amid the politics of large financial institutions, it is much safer to be wrong with a crowd than to be isolated and right. The largest bonuses go to the most articulate cheerleaders for the conventional wisdom.
We understand that we can have knowledge of the kind "Manchester United will probably win" but not knowledge of the kind "the score will be 3-1". This is intrinsic to the nature of a complex system whose outcome depends on many individual uncertain events - whether that system is a sporting contest or the progress of thousands of businesses and millions of individuals. The Treasury and the Bank of England now acknowledge this limitation by indicating a range of projections rather than predicting a number. Perhaps they will soon describe that range as a planning assumption, which is its true status, rather than a forecast.
A decade ago, I observed that academic forecasters, less influenced by the herd, were more extreme - though not necessarily more right - than commercial ones. Today, there are almost no academic forecasters left: the activity no longer commands prestige or attracts research funding.
Yet the number of commercial soothsayers continues to grow. There is supply because there is demand: investors and businesses that continue to believe, despite the evidence, that these people really see the future in their crystal balls. Hope continues to triumph over experience, just as it does among the customers of medical quacks and racing tipsters.
The voices of people who admit they do not know the answer are usually drowned by those who claim, however falsely, that they do. But the future is inherently uncertain, and that is why the only economists worth listening to are those who often say "on the one hand . . . and on the other".
经济算命先生的话不灵
据说哈利?杜鲁门（Harry Truman）总统曾想找一个独臂经济学家。既是独臂，他就不会总说“一方面…另一方面”(译者注: 英文原文为on the one hand… and on the other hand, 字面意思为一只手上…，另一只手上…) 。这笑话说的是经济学家多，其观点更多。但此说并不适用于经济预测家。这些预测家的看法都很一致。他们犯起错误来也很一致。
大约10年前，我在一篇文章中指出，1991年到1994年间，英国的增长率从未进入过预测的范畴。1991和1992年，所有人都高估了增长率。1993和1994年，所有人都低估了增长率。最近，我决定调查后来的预测水平是不是提高了。结果却并不乐观。
英国财政部定期出版经济预测摘要。我看了43份对1999年增长率的预测。这些预测是1998年12月份所作(马上就到1999年了，居然还有人在预测)。预测的增长率平均值为0.8%: 33份预测说增长率在0.3和1.3之间；所有的预测范围在负增长0.5%和正增长2.1之间。而实际上，1999年的增长率为2.8%。
2000年的增长率又被所有的预测团体低估了。这不只是什么悲观不悲观的问题。到了2001年，只有两份预测偏低。
从那以后，经济环境已经稳定了很多，但是经济预测扎堆的现象却更趋极端化。比如，2003年12月，只有3份预测说2004年的增长率在2%和3%之间，而半数以上的预测均把预测定在2.5%和2.9%之间。这些经济预测者观点虽然整齐划一，却并不能说明他们对经济前景都有同样的正确认识。6个月后，他们的观点又出现了类似的一致性，都说2004年的增长率会超过3%。
如此说来，只要看一家预测，就可知道所有预测。几乎所有预测，都是在现状的基础上推测未来的趋势。他们得到结论的过程是用计量经济模拟方法也好，还是凭经验也好，结果都大同小异。最近，由于英国经济的表现开始接近趋势，预测的准确性也有所增加。其实，如果认准1998年之后英国经济增长率保持在2.5%这个长期平均增长水平，其结果会比预测者每次都认同的数字更准确，更接近实际。
预测者之所以殊途同归，是因伦敦金融城的专家们互相观察，互相讨论，这样一来，结论就一致了。出于大型金融机构各自的利益，大家因从众而出现失误，要比保持独立而正确更为安全。谁为常识的呐喊最高，谁的回报也会最丰厚。
大家知道，我们都能预测“曼联队会赢” ，但是我们预测不出“比分是3比1 ” 。所有复杂的体系，其结果会受制于许多不确定的单独事件。预测体育比赛如此，预测成千上万企业和成百上千万个人的发展也是如此，这是所有复杂体系的内在特征。英国财政部和英格兰银行现在都承认有这个局限性，所以都把预测从具体的数字变为一个范围。或许不用多久，他们又会说这范围是预测过程中的一个假设，而不是预测本身。能这样说就对了，实质正是如此。
10年前，我观察到，学术界的预测者不像商业预测者那样从众，他们的观点更极端化，虽然这不说明他们更正确。如今，学术界的预测者几乎荡然无存：预测活动捞不着名，也吸引不了研究基金。
但是，商界的经济算命先生却不断增加。有需求，才有供应：尽管证据确凿，投资人和企业却视而不见。他们依然相信有人能从水晶球里看到未来。希望仍然胜过经验。江湖郎中和马赛预测师之所以仍有市场，也正是这个道理。
也有人会说他们不知道答案，但是更多人宣称他们知道答案，即便他们的答案错得非常离谱。前者的声音会被后者淹没。未来之所以是未来，其本质上就具有不确定性。因此，真正值得倾听的经济学家，反而是说“一方面…但另一方面” 的人。
