| Year | Value | | 2021 | 32.530899474836 | | 2020 | 32.348934961466 | | 2019 | 32.166970448097 | | 2018 | 31.984897774231 | | 2017 | 31.802933876193 | | 2016 | 31.620969978155 | | 2015 | 31.439006080117 | | 2014 | 31.257048945279 | | 2013 | 31.075091810441 | | 2012 | 30.893134675603 | | 2011 | 30.711177540765 | | 2010 | 30.529220405927 | | 2009 | 30.306443977032 | | 2008 | 30.083667548137 | | 2007 | 29.860891119242 | | 2006 | 29.638114690347 | | 2005 | 29.415338261452 | | 2004 | 29.192561832557 | | 2003 | 28.969785403662 | | 2002 | 28.747008974767 | | 2001 | 28.524232545871 | | 2000 | 28.301456116976 | | 1999 | 28.191152969977 | | 1998 | 27.926116079018 | | 1997 | 27.661079188059 | | 1996 | 27.3960422971 | | 1995 | 27.131005406141 | | 1994 | 26.865968515181 | | 1993 | 26.600931624222 | | 1992 | 26.335894733263 | | 1991 | 26.070857842304 | | 1990 | 25.805820951345 |
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