When we convert local currency national accounts data (e.g., Gross Domestic Product) into U.S. dollars, we use a series labeled "DEC alternative conversion factor" rather than the official exchange rate. As a rule, it is the same as the official exchange rate reported in the IMF's International Financial Statistics (IFS), except where further refinements are made by World Bank staff when official exchange rates are deemed unrepresentative of the effective exchange rate during a period. 

In countries where national accounts statistics are reported on a fiscal year basis, the DEC conversion factor is calculated to correspond to the fiscal year. It may thus differ from the official exchange rate reported in IFS since the latter corresponds to the calendar year.

More information on specific differences between the DEC conversion factor and the official exchange rate reported in IFS are available in WDI table 4.16: Exchange rates and prices.