| Year | Value | | 2021 | 5.405 | | 2020 | 4.821 | | 2019 | 7.503 | | 2018 | 7.555 | | 2017 | 6.515 | | 2016 | 6.386 | | 2015 | 5.341 | | 2014 | 5.106 | | 2013 | 4.828 | | 2012 | 4.793 | | 2011 | 4.806 | | 2010 | 4.751 | | 2009 | 4.511 | | 2008 | 4.271 | | 2007 | 4.163 | | 2006 | 4.12 | | 2005 | 4.056 | | 2004 | 4.027 | | 2003 | 4.037 | | 2002 | 3.992 | | 2001 | 3.852 | | 2000 | 3.924 | | 1999 | 3.927 | | 1998 | 3.852 | | 1997 | 3.707 | | 1996 | 3.119 | | 1995 | 4.792 | | 1994 | 4.899 | | 1993 | 5.003 | | 1992 | 4.726 | | 1991 | 4.898 | | 1990 | 4.926 | | 1989 | 4.971 | | 1988 | 4.868 | | 1987 | 4.808 | | 1986 | 4.699 | | 1985 | 4.611 | | 1984 | 4.471 | | 1983 | 4.333 | | 1982 | 4.319 | | 1981 | 4.212 | | 1980 | 4.123 | | 1979 | 4 | | 1978 | 3.95 | | 1977 | 3.81 | | 1976 | 3.72 | | 1975 | 3.63 | | 1974 | 3.475 | | 1973 | 3.4 | | 1972 | 3.32 | | 1971 | 3.3 | | 1970 | 3.2 |
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