| Year | Value | | 2021 | 29.840878601387 | | 2020 | 29.683941676373 | | 2019 | 29.67501074246 | | 2018 | 29.667152400701 | | 2017 | 29.660400576748 | | 2016 | 29.660472402148 | | 2015 | 29.634778519031 | | 2014 | 29.593094920965 | | 2013 | 29.552179100239 | | 2012 | 29.512085271644 | | 2011 | 29.471062615706 | | 2010 | 29.430121648322 | | 2009 | 29.408156115292 | | 2008 | 29.385868476902 | | 2007 | 29.35029145006 | | 2006 | 29.327158367289 | | 2005 | 29.303498910777 | | 2004 | 29.280241329635 | | 2003 | 29.256911208107 | | 2002 | 29.234210491494 | | 2001 | 29.211633655235 | | 2000 | 29.188276916515 | | 1999 | 29.178087420844 | | 1998 | 29.158908124513 | | 1997 | 29.139720618984 | | 1996 | 29.177202474782 | | 1995 | 29.158002433247 | | 1994 | 29.138786965681 | | 1993 | 29.119629276099 | | 1992 | 29.075265560948 | | 1991 | 28.961097621483 | | 1990 | 28.9419667363 |
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