| Year | Value | | 2022 | 2.54 | | 2021 | 2.57 | | 2020 | 2.54 | | 2019 | 2.51 | | 2018 | 2.54 | | 2017 | 2.51 | | 2016 | 2.58 | | 2015 | 2.6 | | 2014 | 2.66 | | 2013 | 2.7 | | 2012 | 2.77 | | 2011 | 2.78 | | 2010 | 2.77 | | 2009 | 2.79 | | 2008 | 2.83 | | 2007 | 2.95 | | 2006 | 3 | | 2005 | 3.08 | | 2004 | 3.42 | | 2003 | 3.48 | | 2002 | 3.62 | | 2001 | 3.66 | | 2000 | 3.75 | | 1999 | 3.9000000954 | | 1998 | 4.1999998093 | | 1997 | 4.4000000954 | | 1996 | 4.5999999046 | | 1995 | 5 | | 1993 | 5.4000000954 | | 1992 | 5.5 | | 1991 | 5.8000001907 | | 1990 | 6 | | 1989 | 6.3000001907349 | | 1988 | 6.5 | | 1987 | 6.5999999046326 | | 1986 | 6.6999998092651 | | 1985 | 6.8000001907349 | | 1984 | 6.8000001907349 | | 1983 | 6.8000001907349 | | 1982 | 6.8000001907349 | | 1981 | 6.9000000953674 | | 1980 | 6.8000001907349 | | 1979 | 6.8000001907349 | | 1978 | 6.9000000953674 | | 1977 | 6.9000000953674 | | 1976 | 6.9000000953674 |
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