| Year | Value | | 2021 | 4.6202996771493 | | 2020 | 4.6915403764834 | | 2019 | 4.7624117467235 | | 2018 | 4.8237427222177 | | 2017 | 4.889033197889 | | 2016 | 4.9625580346189 | | 2015 | 5.0327471018966 | | 2014 | 5.0992495055175 | | 2013 | 5.1762377878799 | | 2012 | 5.2521987154715 | | 2011 | 5.3402810517027 | | 2010 | 5.4227917269079 | | 2009 | 5.4953620216723 | | 2008 | 5.5611826111545 | | 2007 | 5.6296461011444 | | 2006 | 5.6972545961729 | | 2005 | 5.7557251017548 | | 2004 | 5.810055196509 | | 2003 | 5.8719084078772 | | 2002 | 5.9307002221456 | | 2001 | 5.9870764028759 | | 2000 | 6.0470380141229 | | 1999 | 6.1044862731695 | | 1998 | 6.1576142280165 | | 1997 | 6.2123275720358 | | 1996 | 6.2627775865729 | | 1995 | 6.3195553215382 | | 1994 | 6.3697898783411 | | 1993 | 6.4271817516295 | | 1992 | 6.4783054269454 | | 1991 | 6.5282353987086 | | 1990 | 6.5817661036101 | | 1989 | 6.6354204370091 | | 1988 | 6.682894368809 | | 1987 | 6.7318292837008 | | 1986 | 6.7615094385876 | | 1985 | 6.7885034109391 | | 1984 | 6.8160224480865 | | 1983 | 6.8392867031406 | | 1982 | 6.8619702904733 | | 1981 | 6.8890920019244 | | 1980 | 6.9092143931329 | | 1979 | 6.9257513593716 | | 1978 | 6.933523757388 | | 1977 | 6.9377169636107 | | 1976 | 6.9325272205128 | | 1975 | 6.9307367198555 | | 1974 | 6.9279453028538 | | 1973 | 6.9210074752931 | | 1972 | 6.918172648635 | | 1971 | 6.906786115601 | | 1970 | 6.8893830436816 | | 1969 | 6.8736587244579 | | 1968 | 6.8549354052913 | | 1967 | 6.8330592232144 | | 1966 | 6.8120748725369 | | 1965 | 6.7936233349568 | | 1964 | 6.7790947639581 | | 1963 | 6.7647253774169 | | 1962 | 6.744506897867 | | 1961 | 6.7222656154647 | | 1960 | 6.7028204999526 |
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