| Year | Value | | 2021 | 21.341692506016 | | 2020 | 21.364403575112 | | 2019 | 21.387126160193 | | 2018 | 21.409831557236 | | 2017 | 21.432554142317 | | 2016 | 21.455276727398 | | 2015 | 21.477982124441 | | 2014 | 21.500701271915 | | 2013 | 21.523420419388 | | 2012 | 21.546324736376 | | 2011 | 21.5690440791 | | 2010 | 21.601231213674 | | 2009 | 21.681825841731 | | 2008 | 21.762420469787 | | 2007 | 21.843015097844 | | 2006 | 21.9236097259 | | 2005 | 22.004204353957 | | 2004 | 22.084798982013 | | 2003 | 22.16539361007 | | 2002 | 22.245988238126 | | 2001 | 22.326582866183 | | 2000 | 22.407177494239 | | 1999 | 22.521147298552 | | 1998 | 22.635117102865 | | 1997 | 22.749086907177 | | 1996 | 22.86305671149 | | 1995 | 22.977026515803 | | 1994 | 23.090996320116 | | 1993 | 23.204966124428 | | 1992 | 23.318935928741 | | 1991 | 23.432905733054 | | 1990 | 23.546875537366 |
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