| Year | Value | | 2021 | 4.22 | | 2020 | 4.19 | | 2019 | 4.17 | | 2018 | 4.15 | | 2017 | 4.13 | | 2016 | 4.13 | | 2015 | 4.05 | | 2014 | 4.01 | | 2013 | 4.04 | | 2012 | 4.04 | | 2011 | 4.04 | | 2010 | 4.56 | | 2009 | 4.57 | | 2008 | 4.64 | | 2007 | 4.86 | | 2006 | 4.86 | | 2005 | 4.88 | | 2004 | 4.93 | | 2003 | 4.95 | | 2002 | 5 | | 2001 | 5.11 | | 2000 | 5.29 | | 1996 | 5.1300001144 | | 1993 | 6 | | 1990 | 5.4211997986 |
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