| Year | Value | | 2021 | 34.589280036767 | | 2020 | 34.559284726449 | | 2019 | 34.531887037464 | | 2018 | 34.500882027648 | | 2017 | 34.471084022162 | | 2016 | 34.393914226935 | | 2015 | 34.339977841191 | | 2014 | 34.208407899508 | | 2013 | 34.099423226925 | | 2012 | 34.000710294754 | | 2011 | 33.864177886614 | | 2010 | 33.734591672529 | | 2009 | 33.609941948725 | | 2008 | 33.516202785048 | | 2007 | 33.39143984532 | | 2006 | 33.263931274704 | | 2005 | 33.138850796822 | | 2004 | 33.010186946463 | | 2003 | 32.889306401078 | | 2002 | 32.76697613249 | | 2001 | 32.593462803029 | | 2000 | 32.469471994237 | | 1999 | 32.414076632728 | | 1998 | 32.36097217534 | | 1997 | 32.307282018611 | | 1996 | 32.252425374654 | | 1995 | 32.201944457265 | | 1994 | 32.147962863584 | | 1993 | 32.096016406068 | | 1992 | 31.515461970586 | | 1991 | 27.74414533107 | | 1990 | 27.70383350993 |
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