| Year | Value | | 2022 | 3.591 | | 2021 | 4.64 | | 2020 | 4.1 | | 2019 | 3.1 | | 2017 | 4.2 | | 2016 | 4.08 | | 2015 | 3.79 | | 2014 | 3.74 | | 2013 | 3.86 | | 2012 | 3.72 | | 2011 | 3.89 | | 2010 | 4.12 | | 2009 | 5.86 | | 2008 | 3.96 | | 2007 | 3.9 | | 2006 | 4.48 | | 2005 | 5.59 | | 2004 | 5.84 | | 2003 | 5.93 | | 2002 | 5.65 | | 2001 | 3.76 | | 2000 | 5.96 | | 1999 | 4.85 | | 1998 | 3.41 | | 1997 | 2.5 | | 1996 | 3.57 | | 1995 | 3.3 | | 1994 | 3.03 | | 1993 | 3.07 | | 1992 | 3.09 | | 1991 | 2.18 | | 1989 | 2.35 | | 1988 | 3.55 | | 1987 | 5 | | 1986 | 6.84 | | 1985 | 4.27 | | 1984 | 2.69 | | 1983 | 1.69 | | 1982 | 1.5 | | 1981 | 1.65 | | 1980 | 1.8 | | 1979 | 2.12 | | 1978 | 2.26 | | 1977 | 3.93 | | 1976 | 4.48 | | 1975 | 4.55 | | 1974 | 3.97 | | 1973 | 4.51 | | 1972 | 4.7 | | 1971 | 4.8 | | 1970 | 6.06 |
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