| Year | Value | | 2021 | 22.480349550734 | | 2020 | 22.49627438089 | | 2019 | 22.512272627657 | | 2018 | 22.528161297392 | | 2017 | 22.5429541968 | | 2016 | 22.558952443568 | | 2015 | 22.581854043393 | | 2014 | 22.550230111769 | | 2013 | 22.700187783055 | | 2012 | 22.668308847896 | | 2011 | 22.636429912736 | | 2010 | 22.604550977577 | | 2009 | 22.466232188225 | | 2008 | 22.557147639772 | | 2007 | 22.417408771343 | | 2006 | 22.277669902913 | | 2005 | 22.137931034483 | | 2004 | 21.995737558581 | | 2003 | 21.856014282526 | | 2002 | 21.716291006472 | | 2001 | 21.576567730417 | | 2000 | 21.436844454363 | | 1999 | 21.274420935412 | | 1998 | 21.154966592428 | | 1997 | 21.035512249443 | | 1996 | 20.902092143334 | | 1995 | 20.78271756065 | | 1994 | 20.663342977966 | | 1993 | 20.543968395281 | | 1992 | 20.422321130522 | | 1991 | 20.302959830867 | | 1990 | 20.183598531212 |
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