| Year | Value | | 2021 | 6.9531803962461 | | 2020 | 6.8580813347237 | | 2019 | 6.7629822732013 | | 2018 | 6.6678832116788 | | 2017 | 6.5728060479666 | | 2016 | 6.5497393117831 | | 2015 | 6.5266423357664 | | 2014 | 6.5035662148071 | | 2013 | 6.4804900938478 | | 2012 | 6.4574139728884 | | 2011 | 6.4343378519291 | | 2010 | 6.4112617309698 | | 2009 | 6.3858289885297 | | 2008 | 6.3603962460897 | | 2007 | 6.3349635036496 | | 2006 | 6.3095307612096 | | 2005 | 6.2840980187695 | | 2004 | 6.2586652763295 | | 2003 | 6.2332325338895 | | 2002 | 6.2077997914494 | | 2001 | 6.1823670490094 | | 2000 | 6.1569343065693 | | 1999 | 6.1337450469239 | | 1998 | 6.110555265902 | | 1997 | 6.0873654848801 | | 1996 | 6.0641762252346 | | 1995 | 6.0409869655892 | | 1994 | 6.0177971845673 | | 1993 | 5.9946079249218 | | 1992 | 5.9714181438999 |
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