| Year | Value | | 2021 | 6.3448184980848 | | 2020 | 6.3448184980848 | | 2019 | 6.3448184980848 | | 2018 | 6.3448184980848 | | 2017 | 6.2928804863478 | | 2016 | 6.2409424746108 | | 2015 | 6.1890044628738 | | 2014 | 6.2220121586956 | | 2013 | 6.2550198545173 | | 2012 | 6.2880275503391 | | 2011 | 6.3210352461609 | | 2010 | 6.3540429419826 | | 2009 | 6.4146343606143 | | 2008 | 6.4752257792459 | | 2007 | 6.5358171978775 | | 2006 | 6.5964086165091 | | 2005 | 6.6570000351407 | | 2004 | 6.7175914537724 | | 2003 | 6.778182872404 | | 2002 | 6.8387742910356 | | 2001 | 6.8993657096672 | | 2000 | 6.9599571282988 | | 1999 | 6.9419211441824 | | 1998 | 6.9238851600661 | | 1997 | 6.9058491759497 | | 1996 | 6.8878131918333 | | 1995 | 6.8697772077169 | | 1994 | 6.8517412236005 | | 1993 | 6.8337052394841 | | 1992 | 6.8156692553677 | | 1991 | 6.7976332712514 | | 1990 | 6.779597287135 |
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