| Year | Value | | 2021 | 4.585 | | 2020 | 4.693 | | 2019 | 4.809 | | 2018 | 4.932 | | 2017 | 5.052 | | 2016 | 5.174 | | 2015 | 5.306 | | 2014 | 5.463 | | 2013 | 5.618 | | 2012 | 5.784 | | 2011 | 5.935 | | 2010 | 6.071 | | 2009 | 6.174 | | 2008 | 6.251 | | 2007 | 6.379 | | 2006 | 6.481 | | 2005 | 6.572 | | 2004 | 6.653 | | 2003 | 6.702 | | 2002 | 6.779 | | 2001 | 6.81 | | 2000 | 6.832 | | 1999 | 6.875 | | 1998 | 6.887 | | 1997 | 6.906 | | 1996 | 6.894 | | 1995 | 6.917 | | 1994 | 6.928 | | 1993 | 6.932 | | 1992 | 6.96 | | 1991 | 6.983 | | 1990 | 7.037 | | 1989 | 7.072 | | 1988 | 7.105 | | 1987 | 7.119 | | 1986 | 7.081 | | 1985 | 7.035 | | 1984 | 7.004 | | 1983 | 7.04 | | 1982 | 7.086 | | 1981 | 7.115 | | 1980 | 7.134 | | 1979 | 7.157 | | 1978 | 7.178 | | 1977 | 7.209 | | 1976 | 7.231 | | 1975 | 7.23 | | 1974 | 7.223 | | 1973 | 7.222 | | 1972 | 7.215 | | 1971 | 7.204 | | 1970 | 7.186 | | 1969 | 7.163 | | 1968 | 7.187 | | 1967 | 7.148 | | 1966 | 7.116 | | 1965 | 7.085 | | 1964 | 7.053 | | 1963 | 7.023 | | 1962 | 6.997 | | 1961 | 6.968 | | 1960 | 6.936 |
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