| Year | Value | | 2021 | 8.4318046172385 | | 2020 | 8.3732213775105 | | 2019 | 8.3163736650361 | | 2018 | 8.2571206140898 | | 2017 | 8.1984438977004 | | 2016 | 8.1397486557828 | | 2015 | 8.0558329550613 | | 2014 | 8.053071555271 | | 2013 | 7.9613758034278 | | 2012 | 7.8694814134924 | | 2011 | 7.7776273551028 | | 2010 | 7.6844723004781 | | 2009 | 7.5945041232266 | | 2008 | 7.5050914404072 | | 2007 | 7.4157728265257 | | 2006 | 7.3289994264082 | | 2005 | 7.2399626935017 | | 2004 | 7.1508435183422 | | 2003 | 7.0614019551387 | | 2002 | 6.9718103007265 | | 2001 | 6.8827694185281 | | 2000 | 6.7928870436847 | | 1999 | 6.8647200282087 | | 1998 | 6.76784861307 | | 1997 | 6.6709774330042 | | 1996 | 6.5741060178655 | | 1995 | 6.4772346027268 | | 1994 | 6.3803631875882 | | 1993 | 6.2834917724495 | | 1992 | 6.1866205923836 |
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