| Year | Value | | 2020 | 0.2104492249155 | | 2019 | 0.21165029022546 | | 2018 | 0.21472751615019 | | 2017 | 0.21786183855784 | | 2016 | 0.22091733715078 | | 2015 | 0.22386940354475 | | 2014 | 0.22643568717004 | | 2013 | 0.22897337497596 | | 2012 | 0.22979894889961 | | 2011 | 0.2305361232815 | | 2010 | 0.23109632094657 | | 2009 | 0.23147049734908 | | 2008 | 0.23190287907424 | | 2007 | 0.23241166613599 | | 2006 | 0.23249163611339 | | 2005 | 0.23159191651575 | | 2004 | 0.22997017286858 | | 2003 | 0.2285327738851 | | 2002 | 0.22762606500545 | | 2001 | 0.227158115678 | | 2000 | 0.22717101661302 | | 1999 | 0.22777486732114 | | 1998 | 0.22999899950435 | | 1997 | 0.23276248334294 | | 1996 | 0.23587186496807 | | 1995 | 0.23930944865496 | | 1994 | 0.24296317893023 | | 1993 | 0.24685080090742 | | 1992 | 0.25092749073764 | | 1991 | 0.25526060193002 |
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