Measuring energy subsidies using the price gap approach: What does it leave out?

Tracking energy subsidies for a single country is a challenging task; trying to measure them globally is even more so.  Multi-country studies of fossil fuel studies have been done, and normally use a price gap measure.  This approach compares the world price of the energy commodity with a transport-adjusted world reference price at which fuels could be brought in to a country.  While the price gap provides many useful insights on energy subsidy trends, these estimates form a lower-bound for total support.