| Year | Value | | 2021 | 4.01 | | 2020 | 3.42 | | 2019 | 3.54 | | 2018 | 4.04 | | 2017 | 3.7 | | 2016 | 4.91 | | 2015 | 3.8 | | 2013 | 3.86 | | 2012 | 3.9 | | 2011 | 3.98 | | 2010 | 5.05 | | 2009 | 5.05 | | 2008 | 5.07 | | 2007 | 5.03 | | 2006 | 5.12 | | 2005 | 5.14 | | 2004 | 5.16 | | 2003 | 4.48 | | 2002 | 5.29 | | 2001 | 6.93 | | 2000 | 7.19 | | 1999 | 9.45 | | 1998 | 10.29 | | 1997 | 12.75 | | 1996 | 14.29 | | 1995 | 19.79 | | 1994 | 21.22 | | 1993 | 22.04 | | 1992 | 19.52 | | 1990 | 19.54 | | 1989 | 28.37 | | 1988 | 28.36 |
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