| Year | Value | | 2022 | 3.93 | | 2021 | 4.64 | | 2020 | 4.54 | | 2019 | 3.26 | | 2018 | 3.3 | | 2017 | 3.41 | | 2016 | 3.44 | | 2015 | 3.1 | | 2014 | 2.88 | | 2013 | 3.16 | | 2012 | 3.1 | | 2011 | 3.05 | | 2010 | 3.39 | | 2009 | 3.66 | | 2008 | 3.32 | | 2007 | 3.23 | | 2005 | 3.53 | | 2004 | 3.54 | | 2003 | 3.61 | | 2002 | 3.48 | | 2001 | 3.53 | | 2000 | 3 | | 1999 | 3.43 | | 1998 | 3.2 | | 1997 | 2.45 | | 1996 | 2.52 | | 1995 | 3.15 | | 1993 | 4.11 | | 1992 | 3.71 | | 1990 | 4.5 | | 1989 | 5.73 | | 1988 | 7.26 | | 1987 | 7.33 | | 1986 | 8.29 | | 1985 | 6.92 | | 1984 | 5.84 | | 1983 | 3.8 | | 1982 | 3.4 |
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