| Year | Value | | 2021 | 4.257 | | 2020 | 4.323 | | 2019 | 4.385 | | 2018 | 4.442 | | 2017 | 4.503 | | 2016 | 4.56 | | 2015 | 4.594 | | 2014 | 4.655 | | 2013 | 4.723 | | 2012 | 4.806 | | 2011 | 4.917 | | 2010 | 5.007 | | 2009 | 5.034 | | 2008 | 5.052 | | 2007 | 5.092 | | 2006 | 5.078 | | 2005 | 5.069 | | 2004 | 5.075 | | 2003 | 5.145 | | 2002 | 5.194 | | 2001 | 5.23 | | 2000 | 5.269 | | 1999 | 5.335 | | 1998 | 5.384 | | 1997 | 5.441 | | 1996 | 5.543 | | 1995 | 5.646 | | 1994 | 5.745 | | 1993 | 5.845 | | 1992 | 5.947 | | 1991 | 6.049 | | 1990 | 6.133 | | 1989 | 6.219 | | 1988 | 6.302 | | 1987 | 6.385 | | 1986 | 6.461 | | 1985 | 6.535 | | 1984 | 6.616 | | 1983 | 6.696 | | 1982 | 6.772 | | 1981 | 6.865 | | 1980 | 6.936 | | 1979 | 6.999 | | 1978 | 7.063 | | 1977 | 7.111 | | 1976 | 7.144 | | 1975 | 7.153 | | 1974 | 7.15 | | 1973 | 7.128 | | 1972 | 7.118 | | 1971 | 7.103 | | 1970 | 7.066 | | 1969 | 7.039 | | 1968 | 6.976 | | 1967 | 6.959 | | 1966 | 6.935 | | 1965 | 6.902 | | 1964 | 6.873 | | 1963 | 6.85 | | 1962 | 6.803 | | 1961 | 6.753 | | 1960 | 6.717 |
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