| Year | Value | | 2021 | 26.708309071339 | | 2020 | 26.917050447303 | | 2019 | 27.125791823267 | | 2018 | 27.334533199231 | | 2017 | 27.543274575194 | | 2016 | 27.752015951158 | | 2015 | 27.960757327122 | | 2014 | 28.169498703086 | | 2013 | 28.37824007905 | | 2012 | 28.586981455014 | | 2011 | 28.795722830978 | | 2010 | 29.004464206942 | | 2009 | 29.213205582905 | | 2008 | 29.421946958869 | | 2007 | 29.630688334833 | | 2006 | 29.839429710797 | | 2005 | 30.048171086761 | | 2004 | 30.256912462725 | | 2003 | 30.465653838689 | | 2002 | 30.674395214652 | | 2001 | 30.883136590616 | | 2000 | 31.09187796658 | | 1999 | 31.300619342544 | | 1998 | 31.509360718508 | | 1997 | 31.718102094472 | | 1996 | 31.926843470436 | | 1995 | 32.1355848464 | | 1994 | 32.344326222363 | | 1993 | 32.553067598327 | | 1992 | 32.761808974291 | | 1991 | 32.970550350255 | | 1990 | 33.179291726219 |
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