Specialists at the World Trade Organisation agricultural liberalisation talks in Geneva are calculating commonly agreed values for the various kinds of duties paid on traded foodstuffs.

These can be so many US dollars per tonne of sugar, or euros per 100 chickens. But to simplify a deal on reducing these tariffs by certain percentages worldwide, one common way of valuing these charges is needed, so experts are converting duties into virtual ‘ad valorem’ tariffs set at a proportion of an item’s value.

The aim is to decide which existing duties are ‘high’, ‘medium’ and ‘low’. WTO member governments have agreed a Doha deal will hammer the highest tariffs.