What does "euro area (changing composition)" mean for time series data from Eurostat and the European Central Bank?
The euro zone is an aggregate of countries, but not a static one; since its establishment in 1999, it has expanded incrementally from 11 member states to 19. Some datasets retroactively apply a constant membership definition (and are hence subject to revision) but others describe a different population at different times.
For example, ECB daily time series IR%GBAAA10YUD.IEUZN is described in Data Buffet as:
Interest Rates: Euro area (changing composition) - Central government bond; nominal; AAA-rated - Svensson model - continuous compounding - yield error minimisation - Yield curve spot rate - 10-Year Maturity, (% Per Annum, NSA)
The phrase "changing composition" indicates that the yield is derived from the membership at a particular time. So, for periods in 2005 the yield was computed from 12 governments, but 2010 used 16.
ECB also reports "fixed composition" series.
April 2015 - Data Buffet will soon adopt a new geo code for "changing composition" series, IEZCC. For convenience and backwards compatibility, geo code IEUZN will be used as an alias, either to the IEZCC series (if it is the only reported series) or to the source's chosen key series (if there are multiple variants).
Euro Area Yield Curves