| Year | Value | | 2022 | 4.992 | | 2021 | 6.459 | | 2020 | 5.201 | | 2019 | 4.56 | | 2018 | 4.933 | | 2017 | 5.561 | | 2016 | 6.064 | | 2015 | 5.802 | | 2014 | 5.674 | | 2013 | 5.367 | | 2012 | 4.909 | | 2011 | 4.637 | | 2010 | 4.883 | | 2009 | 5.371 | | 2008 | 4.198 | | 2007 | 4.909 | | 2006 | 5.32 | | 2005 | 5.682 | | 2004 | 5.969 | | 2003 | 4.785 | | 2002 | 4.86 | | 2001 | 6.4 | | 2000 | 4.694 | | 1999 | 4.707 | | 1998 | 5.493 | | 1997 | 5.157 | | 1996 | 5.291 | | 1995 | 4.353 | | 1994 | 3.535 | | 1993 | 4.25 | | 1992 | 3.59 | | 1991 | 5.86 | | 1990 | 3.25 | | 1989 | 3.14 | | 1988 | 3.55 | | 1987 | 3.79 | | 1986 | 3.12 | | 1985 | 3.6 | | 1984 | 3.8 | | 1983 | 4.11 | | 1982 | 3.35 | | 1981 | 2.06 | | 1980 | 1.9 | | 1979 | 2 | | 1978 | 2.1 | | 1977 | 1.8 | | 1976 | 2 | | 1975 | 2 | | 1974 | 1.3 | | 1973 | 1.2 | | 1972 | 1.3 | | 1971 | 1.5 | | 1970 | 2.4 | | 1969 | 2.8 |
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