| Year | Value | | 2020 | 2.743801491517 | | 2019 | 2.5560341960285 | | 2018 | 2.5929354696068 | | 2017 | 2.6265243162295 | | 2016 | 2.6613627351828 | | 2015 | 2.708967785061 | | 2014 | 2.8122225290764 | | 2013 | 2.6895715509128 | | 2012 | 2.8086127011629 | | 2011 | 2.8659591277365 | | 2010 | 2.9283085085641 | | 2009 | 3.1672246270195 | | 2008 | 3.210678987471 | | 2007 | 3.2602818699142 | | 2006 | 3.3142979508212 | | 2005 | 3.2170145580498 | | 2004 | 3.262483075869 | | 2003 | 3.2304281002665 | | 2002 | 3.3978402818401 | | 2001 | 3.3370563598423 | | 2000 | 3.4189864878338 | | 1999 | 3.4845003517254 | | 1998 | 3.1638211740325 | | 1997 | 3.2819082681132 | | 1996 | 2.0694205898438 | | 1995 | 2.1207720443448 | | 1994 | 1.6959939008829 | | 1993 | 1.7622675852277 | | 1992 | 1.8338059507294 | | 1991 | 1.8994955783964 |
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