| Year | Value | | 2021 | 6.4695009242144 | | 2020 | 6.4695009242144 | | 2019 | 6.4695009242144 | | 2018 | 6.4695009242144 | | 2017 | 6.4695009242144 | | 2016 | 6.4695009242144 | | 2015 | 7.6247689463956 | | 2014 | 7.5231053604436 | | 2013 | 7.4214417744917 | | 2012 | 7.3197781885397 | | 2011 | 7.2181146025878 | | 2010 | 7.1164510166359 | | 2009 | 7.1118299445471 | | 2008 | 7.1072088724584 | | 2007 | 7.1025878003697 | | 2006 | 7.097966728281 | | 2005 | 7.0933456561922 | | 2004 | 7.0887245841035 | | 2003 | 7.0841035120148 | | 2002 | 7.0794824399261 | | 2001 | 7.0748613678373 | | 2000 | 7.0702402957486 | | 1999 | 6.9731977818854 | | 1998 | 6.8761552680222 | | 1997 | 6.779112754159 | | 1996 | 6.6820702402957 | | 1995 | 6.5850277264325 | | 1994 | 6.4879852125693 | | 1993 | 6.3909426987061 | | 1992 | 6.2939001848429 | | 1991 | 6.1968576709797 | | 1990 | 6.0998151571165 |
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