HAVING raised the alarm on deflation, the Federal Reserve has now begun to sound the all clear. The statement it released after its policy meeting on June 24th notably omitted the warning from its three prior meetings that “inflation could persist for a time below rates that best foster economic growth and price stability”. To be sure, with the economy gradually finding a bottom and the rate of decline in home prices slowing, the chances of a downward spiral of deflation and economic activity have diminished. Yet it seems premature to write off the threat as long as a large output gap persists. The output gap is the difference between actual economic output and the most the economy could produce given the capital, know-how and people available. When actual output exceeds potential, demand for products and labour bids up prices and wages, fuelling inflation. When actual output falls short, competition for scarce sales and jobs puts downward pressure on inflation.

Estimating how big the output gap is, and how much of a deflationary threat it still poses, is not easy. The Congressional Budget Office (CBO) estimates that the gap topped 6% in the first quarter of this year and will average more than 7% in 2009, which would be the largest figure on record. Given that core inflation was so low when the recession began, it is not a stretch to believe that, with so much slack in the economy, it could yet turn negative. But this view has been challenged in a note by John Williams and Justin Weidner of the Federal Reserve Bank of San Francisco. Rather than follow the conventional route of deriving an inflation forecast from an estimate of potential output, they do the opposite: they infer the output gap from the behaviour of inflation. As in the euro zone, where consumer prices fell for the first time ever in the 12 months to June, and Japan, where inflation excluding perishable food was -1.1% in May, inflation in America is now negative because of a drop in fuel prices last year. But core inflation is 1.8%, within its range this decade. The authors take this as evidence that the output gap may have been only 2% in the first quarter, implying little or no threat of deflation.

NAIRU's non-alignment

This divergence in estimates highlights the biggest problem in relying on the output gap: it is a slippery thing to measure. How do you really gauge a firm's capacity, especially in services? How many of those not working could work? How fast is productivity growing over time? Economic shocks make the task even harder. They may render big chunks of the capital stock obsolete: many idle car factories, for example, may never reopen. Workers thrown out of a shrunken industry like finance or construction may take years to retrain for another. Some may never succeed. Although unemployed, they are not really competing for the jobs that fall vacant and are thus not putting much downward pressure on wages. That means the “non-accelerating inflation rate of unemployment”, or the Nairu, may have risen. In other words, when actual output falls, it can drag potential output down with it—the main reason why Mr Williams and Mr Weidner believe that the gap is smaller than the CBO's estimates.

That recessions can reduce potential output is not controversial. The question is: by how much? In its latest Economic Outlook, the OECD concludes that the collapse in business investment will cause potential output to grow more slowly in America this year and next but that it will not fall. It does think the Nairu will rise measurably in the euro area, where relatively rigid labour markets mean someone who loses a job will take much longer to find another, during which time his skills atrophy. But in the United States, whose job market is more flexible, the Nairu will barely rise (see chart).

If Mr Williams and Mr Weidner are wrong and the output gap is large, then there are other explanations for why American inflation has not fallen further. The simplest is that not enough time has elapsed. Chris Varvares of Macroeconomic Advisers, a forecasting consultancy, thinks that core inflation will fall to close to zero in 2011. While it has been firm so far this year, he argues it will fall noticeably later this year as residual seasonal factors recede.

Even if inflation were to fall to between zero and 1.5%, say, that would be a small drop given the CBO's estimate of the output gap. A comparable gap in 1981-83 produced a drop in core inflation of six percentage points. But in the early 1980s, inflation had fluctuated so much for so long that workers and firms quickly adapted their wage- and price-setting behaviour to the latest trends, so inflation responded more swiftly to falling demand. Since then, inflationary expectations have stabilised at around 2%, which means that inflation responds more sluggishly to demand (as the recessions of 1990-91 and 2001 demonstrated). The OECD notes that when Finland and Canada experienced large and persistent output gaps in the 1990s, inflation fell quite far but did not become deflation, which it attributes in part to the success of central banks in anchoring expectations with inflation targets.

Such expectations may rise if investors worry that central banks will print money to finance governments' rising fiscal deficits. But even if they remain well anchored, expectations alone do not explain why inflation does not respond more to economic slack. A large and persistent output gap in the 1990s eventually tipped Japan into deflation in 2000; inflation expectations also turned negative. As long as the gap persisted, deflation should have accelerated. It did not, ranging from -0.3% to -0.9% between 2000 and 2003. According to Ken Kuttner, an economist at Williams College, Japan's output gap may have been smaller than thought; workers may have resisted pay cuts; and perhaps most important, at low rates of either inflation or deflation firms may change prices less frequently, reducing the impact of output gaps.

If so, this would provide a buffer to deflation in the rich world today, despite the presence of large gaps. But the thought is not entirely comforting. It also means that if inflation does fall to zero or turns slightly negative, it could be difficult to get it back up. The best cure for deflation remains prevention.