| Year | Value | | 2021 | 5.978 | | 2020 | 5.985 | | 2019 | 6.008 | | 2018 | 6.037 | | 2017 | 6.048 | | 2016 | 6.04 | | 2015 | 6.035 | | 2014 | 6.024 | | 2013 | 6.017 | | 2012 | 6.006 | | 2011 | 5.963 | | 2010 | 5.937 | | 2009 | 5.922 | | 2008 | 5.914 | | 2007 | 5.892 | | 2006 | 5.867 | | 2005 | 5.849 | | 2004 | 5.827 | | 2003 | 5.852 | | 2002 | 5.836 | | 2001 | 5.851 | | 2000 | 5.915 | | 1999 | 5.933 | | 1998 | 5.934 | | 1997 | 5.96 | | 1996 | 5.989 | | 1995 | 5.972 | | 1994 | 5.933 | | 1993 | 5.94 | | 1992 | 5.975 | | 1991 | 6.032 | | 1990 | 6.045 | | 1989 | 6.06 | | 1988 | 6.081 | | 1987 | 6.102 | | 1986 | 6.093 | | 1985 | 6.106 | | 1984 | 6.111 | | 1983 | 6.124 | | 1982 | 6.141 | | 1981 | 6.139 | | 1980 | 6.106 | | 1979 | 6.071 | | 1978 | 6.018 | | 1977 | 5.968 | | 1976 | 5.932 | | 1975 | 5.889 | | 1974 | 5.911 | | 1973 | 5.928 | | 1972 | 5.96 | | 1971 | 5.986 | | 1970 | 6.002 | | 1969 | 6.037 | | 1968 | 6.06 | | 1967 | 6.015 | | 1966 | 5.983 | | 1965 | 5.941 | | 1964 | 5.906 | | 1963 | 5.868 | | 1962 | 5.851 | | 1961 | 5.828 | | 1960 | 5.814 |
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