| Year | Value | | 2021 | 2.18 | | 2020 | 2.31 | | 2019 | 5.91 | | 2018 | 3.77 | | 2017 | 3.69 | | 2016 | 3.51 | | 2015 | 3.52 | | 2014 | 3.77 | | 2013 | 2.33 | | 2012 | 3.8 | | 2011 | 3.84 | | 2010 | 3.63 | | 2009 | 3.93 | | 2008 | 3.92 | | 2007 | 4.11 | | 2006 | 4.35 | | 2005 | 4.05 | | 2004 | 4.17 | | 2003 | 4.25 | | 2002 | 4.41 | | 2001 | 4.43 | | 2000 | 4.25 | | 1999 | 3.88 | | 1998 | 3.89 | | 1997 | 4.01 | | 1996 | 4.25 | | 1995 | 4.04 | | 1994 | 4.12 | | 1993 | 4.03 | | 1992 | 3.97 | | 1991 | 3.97 | | 1990 | 4 | | 1989 | 4.41 | | 1988 | 4.55 |
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