ON THE morning of December 7th 1941, George Elliott Junior noticed “the largest blip” he had ever seen on a radar near America’s naval base at Pearl Harbour. His discovery was dismissed by his superiors, who were thus unprepared for the Japanese bombers that arrived shortly after. The mistake prompted urgent research into “receiver operating characteristics”, the ability of radar operators to distinguish between true and false alarms.

A similar concern motivates research at the Bank for International Settlements (BIS) in Basel, Switzerland. Its equivalent to the radar is a set of economic indicators that can potentially detect the approach of financial crises. A prominent example is the “credit gap”, which measures the divergence between the level of credit to households and non-financial firms, expressed as a percentage of GDP, and its long-term trend. A big gap may reflect the kind of unsustainable credit boom that often precedes a crisis. Anything above 9% of GDP is reason to worry, according to Iñaki Aldasoro, Claudio Borio and Mathias Drehmann of the BIS.

The biggest blips on the oscilloscope include Canada (9.6%), Singapore (11.1%) and Switzerland (16.3%), according to the latest readings, released on March 11th. But the one that has kept everyone’s eyes peeled is China, with a gap of 16.7% in the third quarter of 2017 (the latest BIS figure available).

As a crisis-detector, the credit gap has some appealing operating characteristics. It can stand alone. It can be estimated quarterly across many economies. And, according to Mr Aldasoro and his co-authors, it would have predicted 80% of the crises since 1980 in the countries and periods for which data are available.

The problem is that it has also predicted many crises that never arrived. When such early-warning indicators flash red, the chance of a crisis in the next three years is “around 50%”, says Mr Drehmann. The BIS provides over 5,200 credit-gap readings for the period since 1980. In over 850 instances, the gap exceeded 9% but skies remained clear.