| Year | Value | | 2021 | 10.093677261202 | | 2020 | 9.3448711499537 | | 2019 | 9.9071691866066 | | 2018 | 9.691023944961 | | 2017 | 10.06163529229 | | 2016 | 9.4046031407415 | | 2015 | 9.5561790166475 | | 2014 | 9.0873395319748 | | 2013 | 9.2562140987697 | | 2012 | 9.1690747420643 | | 2011 | 9.3219521882829 | | 2010 | 9.3640080389708 | | 2009 | 9.3485279867304 | | 2008 | 9.587141895076 | | 2007 | 9.0656237703257 | | 2006 | 9.19403806543 | | 2005 | 9.1261607283783 | | 2004 | 9.020393896535 | | 2003 | 8.9775771862927 | | 2002 | 8.9557042716729 | | 2001 | 8.9706326300942 | | 2000 | 10.032345418206 | | 1999 | 8.9188865324647 | | 1998 | 9.261956958868 | | 1997 | 8.3239292123457 | | 1996 | 8.3380450682087 | | 1995 | 7.9154614857455 | | 1994 | 7.8557463596743 | | 1993 | 7.3396514658956 | | 1992 | 7.9193584466956 | | 1991 | 7.7608400962141 | | 1990 | 7.9537221560364 | | 1989 | 7.8248655333472 | | 1988 | 8.3386018448151 | | 1987 | 7.9666436975068 | | 1986 | 8.309256435845 | | 1985 | 7.6720894475931 | | 1984 | 7.7927688975841 | | 1983 | 7.635406112469 | | 1982 | 7.600636453227 | | 1981 | 7.3075288588539 | | 1980 | 7.2921979583024 |
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