| Year | Value | | 2020 | 1.9911684023018 | | 2019 | 2.4602856084056 | | 2018 | 2.5020955049854 | | 2017 | 2.5560942879281 | | 2016 | 2.5560339441308 | | 2015 | 2.0522742417853 | | 2014 | 2.0230484722414 | | 2013 | 2.0561365597207 | | 2012 | 2.0728134662969 | | 2011 | 2.0930599058369 | | 2010 | 2.1124310353397 | | 2009 | 1.8351881537324 | | 2008 | 1.8679279379466 | | 2007 | 1.9106763260982 | | 2006 | 1.9850099435149 | | 2005 | 2.0438855500144 | | 2004 | 2.1551175224769 | | 2003 | 2.0213057635232 | | 2002 | 2.1112040133779 | | 2001 | 2.3160359757588 | | 2000 | 2.4356305689678 | | 1999 | 2.4904032599958 | | 1998 | 2.528113960331 | | 1997 | 2.5684554531979 | | 1996 | 3.5176144543803 | | 1995 | 3.6095591439157 | | 1992 | 2.7547146941991 | | 1991 | 2.7806279213972 |
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