| Year | Value | | 2021 | 4.126 | | 2020 | 3.997 | | 2019 | 4.051 | | 2018 | 4.013 | | 2017 | 4.025 | | 2016 | 3.986 | | 2015 | 3.87 | | 2014 | 3.91 | | 2013 | 3.895 | | 2012 | 3.832 | | 2011 | 3.856 | | 2010 | 3.784 | | 2009 | 3.761 | | 2008 | 3.761 | | 2007 | 3.773 | | 2006 | 3.706 | | 2005 | 3.707 | | 2004 | 3.687 | | 2003 | 3.624 | | 2002 | 3.563 | | 2001 | 3.506 | | 2000 | 3.442 | | 1999 | 4.115 | | 1998 | 4.069 | | 1997 | 3.994 | | 1996 | 3.896 | | 1995 | 3.797 | | 1992 | 5.017 | | 1991 | 4.874 | | 1990 | 4.695 |
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