| Year | Value | | 2021 | 27.962355212355 | | 2020 | 28.179536679537 | | 2019 | 28.396718146718 | | 2018 | 28.6138996139 | | 2017 | 28.831081081081 | | 2016 | 29.048262548262 | | 2015 | 29.265444015444 | | 2014 | 29.482625482626 | | 2013 | 29.699806949807 | | 2012 | 29.916988416988 | | 2011 | 30.13416988417 | | 2010 | 30.351351351351 | | 2009 | 30.568532818533 | | 2008 | 30.785714285714 | | 2007 | 31.002895752896 | | 2006 | 31.220077220077 | | 2005 | 31.437258687259 | | 2004 | 31.65444015444 | | 2003 | 31.871621621622 | | 2002 | 32.088803088803 | | 2001 | 32.305984555985 | | 2000 | 32.523166023166 | | 1999 | 32.740347490348 | | 1998 | 32.957528957529 | | 1997 | 33.17471042471 | | 1996 | 33.391891891892 | | 1995 | 33.609073359073 | | 1994 | 33.826254826255 | | 1993 | 34.043436293436 | | 1992 | 34.260617760618 | | 1991 | 34.477799227799 | | 1990 | 34.694980694981 |
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