Euro-zone series: Until December 1998 it is an aggregate of interbank deposit bid rates weighted by country GDP (Gross Domestic Product). Thereafter the rate is the EONIA (Euro OverNight Index Average), the effective overnight reference rate for the euro, computed as a weighted average of all overnight unsecured lending transactions in the interbank market, initiated within the euro area by the contributing panel banks. EONIA is computed with the help of the European Central Bank. EU15 series: Until December 1998, this is a theoretical rate based on an aggregation of day-to-day rates weighted by country GDP. Thereafter the rate is an average of the EONIA and the rates of the non-euro-zone countries, weighted by country GDP. National series: broadly speaking, these are day-to-day interbank rates. Source: European Central Bank.