In few industries do statisticians have to behave like spies, but those charged with collecting data on oil supplies have to rely on secret networks of informers in terminals around the world, who monitor tanker timetables and scrutinise shipping movements to report on changes in import and export volumes.

This reliance on informal and often unreliable sources about the basics of supply and demand has led to disparities between oil statisticians' estimates of production. Debates rage in the industry about the true strength of consumption growth and, for example, how much of China's rise in demand has gone into building stockpiles. The need for accurate information has become more critical as prices have risen, playing havoc with markets and economies around the world, and calls for an overhaul of the system are growing.