2009年以來，央行行長們為了支撐經濟增長，採取了很多的措施，比如：低利率、量化寬松、前瞻性指引以及當前的負利率政策。
但行長們的日子比以前艱難多了，2008年金融危機之前的經濟關系已經難以適用於當前的時代了。舉例來講，失業率和通脹之間的關系是用菲爾普斯曲線來衡量的。失業率下降，企業對勞動力的競争以及工資水平都會增加，通脹也會開始增加。這兩者的關系是基於“前瞻性指引”的。2013年8月，英格蘭銀行行長Mark Carney表示，失業率跌至7%以下是緊縮政策的必要條件（但不是充分條件）。英國失業率下降至5.1%，但通脹率只有0.3%，所以沒有必要採取緊縮政策。
2012年12月，美聯儲在Ben Bernanke的領導下引入了前瞻性指引。美聯儲認為6.5%的失業率可能是個契機，但直到2015年12月失業率跌至5%時，利率的上行趨勢才開始出現。
這兩家央行到底發生了什麼事？最好的解釋是非加速性失業增長率(NAIRU)。NAIRU並不是一組數字，它在每個週期都不一樣。Paul Krugman指出，隨著失業率的下降，美聯儲的預期NAIRU也會隨之下降。
這取決於勞動力市場還有多少剩餘產能。最佳的計算方法不是失業率，因為失業率統計的是有保險的人，而不是參與勞動的人。在美國，勞動力參與率由2000年的峰值67%下降至62.9%。值得註意的是，隨著失業率的下降，勞動力參與率最好是上升的，但如圖所示，最近兩組數據皆有下降。如果薪水增加了，這些人會重返勞動力市場嗎？這個很難說，有些人失業久了，再找新工作就會變難，另外一些人可能是提前退休的。
勞動力參與率沒有絕對的標準，在20世紀50年代至60年代，女人們不參加工作，那時的數據是低於60%的。
所以，想要計算勞動力市場的寬松程度，還有其他的辦法嗎？經濟學家試圖用實際產出和潛在產出的“產出缺口”來計算。若要計算產出缺口，就要考慮增長率趨勢，但這個概念是捉摸不定的。2008年以來，大多數國家的增長率都呈下降趨勢。英國預算責任辦公室最近修改了增長預期，因為產出能力過低。
十二月以來最大的預期變動是降低了潛在產出增長率。潛在產出增長率是每工作一小時所獲得的持續性產量，這是潛在規模的關鍵驅動因素。該數據自十二月開始，展示了2015年中期以來的產出增速，與我們假設的產出會回到危機前的水平相一致。隨著危機後產出增長疲軟期的延續，我們越來越註重對未來預期的引導。在過去的十年中，我們每年對GDP增長的預期已經下調了0.3%。
為什麼產出增長會趨於疲軟？1870至1970年間，美國的轉型變革大大延長了人們的壽命，也提高了人們的生活質量。人們可以在家中獲得幹淨的飲用水，降低了發病率，也減少了洗衣服的苦差事。1970年以來，信息技術領域的變革是無比顯著的，但其影響力卻遠低於上一個時代。在過去的40年中，公路運輸（擁堵）和航空運輸（安檢）並沒有做到真正的提速。冰箱、洗衣機、空調等重要家電早已在1970年之前完全普及。
顯而易見，IT技術的進步可能會提高產出，我們在分銷和零售領域都已見識到了。但為什麼最近幾年不見產出的增長呢？原因之一是收入增長過慢，雇佣員工比投資新技術要便宜得多。另一原因是當代經濟的重心已由制造業轉向服務業，而服務業的產出是很難提高的。10分鐘理出的頭發肯定比不上一個小時做出的發型。在網絡方面，頻繁的郵件、推特訂閱、搞笑視頻等都會降低辦公室一族的工作效率。換句話說，我們很難預測服務業的產出增長，也許實際的GDP數據會高於我們的預期。這是未來要擔心的事情，數據公佈之後，我們就要想辦法解釋為什麼產出降低了。
此外，各國央行也不知道經濟增長會有多快，低失業率何時會到來通脹壓力。他們有時也會迷茫。
The central bankers’ dilemma
CENTRAL banks have taken the brunt of supporting the global economy, ever since 2009, the last time that governments made any concerted attempt to indulge in a fiscal status. Low interest rates, quantitative easing, forward guidance and now negative rates have all been wheeled out.
But life for a central banker is more difficult than it used to be; the economic relationships that prevailed before the 2008 crisis may not hold good today. Take the relationship between unemployment and inflation, famously linked by the Phillips curve. As unemployment falls, inflation should start to rise as businesses compete for labour and wages go up. The relationship was the basis for the introduction of "forward guidance". In August 2013, Mark Carney, governor of the Bank of England, said that a fall in unemployment below 7% would be a necessary (but not sufficient) condition for tighter policy. UK unemployment has fallen to 5.1% but inflation is only 0.3% and there has been no need to tighten policy.
The Federal Reserve had introduced forward guidance under Ben Bernanke way back in December 2012. That envisioned a US unemployment rate of 6.5% as a possible trigger; in fact, the first upward move in rates did not occur until December 2015, when the jobless rate had fallen all the way down to 5%.
So what has been happening? The economic concept underlying the approach of the two central banks is that of the NAIRU; the non-accelerating inflation rate of unemployment. But it seems clear that the NAIRU is not a set figure; it varies from cycle to cycle. Paul Krugman has shown that the Fed's estimate of the NAIRU has trickled down as unemployment has fallen.
It all depends on how much spare capacity there is in the labour market. The best measure might not be the unemployment rate, which measures those entitled to claim benefits, but the labour force participation rate, which calculates the proportion of those of working age who are in employment. In the US, that has fallen from a peak of 67% reached in 2000 to 62.9%. What is remarkable about the chart is that one would expect the labour force participation rate to rise as unemployment falls; until recently, the two have dropped in tandem. Would some of those people rejoin the labour market if wages were higher? It is hard to know; some may have been jobless for a time and thus may find it difficult to get jobs, others may have retired early. There is no "right" level of labour force participation that can be determined; the rate was below 60% for much of the 1950s and 1960s before more women joined the workforce.
So if you can't know how much slack there is in the labour market, are there other measures? Economists use the idea of the "output gap", the difference between the economy's actual output and its potential production. To figure this out, they need an idea off thetrend rate of growth. But that is a slippery concept. It looks as if, in most developed countries, the trend rate of growth has fallen since 2008. In Britain, the Office for Budgetary Responsibility recently revised down its assumed rate of growth because of poor productivity.
The most significant forecast change we have made since November has been to revise down potential productivity growth. This is the amount of output the economy can produce sustainably per hour worked and is a key driver of its potential size. The data available in November showed a pick-up in productivity growth in mid-2015, consistent with our assumption that the receding financial crisis would exert less of a drag and that trend productivity growth would return to its pre-crisis average rate by the end of the forecast. But more recent data suggest that this was another false dawn. With the period of weak productivity growth post-crisis continuing to lengthen, we have placed more weight on that as a guide to future prospects – although this judgement remains highly uncertain. This in turn has prompted us to revise down our GDP growth forecasts by around 0.3 percentage points a year to an average of 2.1 per cent a year over the rest of the decade.
Why would productivity growth be weaker? Mr Gordon displays, in impressive detail, the transformational changes in America between 1870 and 1970 that brought vast improvements to both life expectancy and the quality of life; people have clean drinking water in their houses, for example, reducing the incidence of disease and the drudgery of clothes washing. The changes in the economy since 1970 have been remarkable in the sectors where they have occurred; information technology and communications. But they are nothing like as widespread as in the previous era. Conversely, there has been no improvement in the real speed of road transport (congestion) or air transport (security checks) in the last 40 years. Vital household gadgets such as fridges, washing machines and central heating were all widespread before 1970.
Clearly, improvements in IT have the potential to improve productivity; we have seen it happen in areas such as distribution and retail. So why hasn't it happened in recent years? One answer might be that wage growth has been so low; it is cheaper to employ workers than to invest in new technology. Another answer could be that modern economies are service- rather than manufacturing-dominated and it is simply harder to improve service productivity; a 10-minute haircut is not better (and probably worse) than an hour-long appointment. Perhaps the internet, with the instant distractions of e-mails, Twitter feeds and cat videos, actually reduces productivity for office workers. Alternatively, we may simply be bad at measuring the improvement in service-sector productivity and later revisions may show that GDP is higher (and inflation lower) than we thought. We may discover this is the case in future, although it will need to be a big revision to explain the productivity slowdown.
In the meantime, central banks simply don't know the answer to these questions - how fast can the economy grow and, at what point will lower unemployment cause inflationary pressures? No wonder they sometimes seem adrift.
本文翻譯由兄弟財經提供
文章來源：http://www.economist.com/blogs/buttonwood/2016/03/monetary-policy