| Year | Value | | 2021 | 2.79 | | 2020 | 2.87 | | 2019 | 8.7 | | 2018 | 3.25 | | 2017 | 3.36 | | 2016 | 2.76 | | 2015 | 2.8 | | 2014 | 2.93 | | 2013 | 2.87 | | 2012 | 2.99 | | 2011 | 3.03 | | 2010 | 2.96 | | 2009 | 3 | | 2008 | 3.1 | | 2007 | 3.01 | | 2006 | 3.14 | | 2005 | 3.23 | | 2004 | 3.31 | | 2003 | 3.53 | | 2002 | 3.98 | | 2001 | 3.82 | | 2000 | 3.87 | | 1999 | 3.88 | | 1998 | 4.49 | | 1997 | 4.34 | | 1996 | 4.44 | | 1995 | 4.59 | | 1993 | 5.12 | | 1992 | 5.25 | | 1991 | 5.31 | | 1990 | 5.2 | | 1989 | 5.28 |
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