| Year | Value | | 2021 | 2.83 | | 2020 | 2.873 | | 2019 | 2.918 | | 2018 | 2.973 | | 2017 | 3.027 | | 2016 | 3.077 | | 2015 | 3.175 | | 2014 | 3.31 | | 2013 | 3.436 | | 2012 | 3.559 | | 2011 | 3.681 | | 2010 | 3.778 | | 2009 | 3.842 | | 2008 | 3.892 | | 2007 | 3.847 | | 2006 | 3.802 | | 2005 | 3.779 | | 2004 | 3.801 | | 2003 | 3.838 | | 2002 | 3.832 | | 2001 | 3.853 | | 2000 | 3.921 | | 1999 | 4.037 | | 1998 | 4.164 | | 1997 | 4.337 | | 1996 | 4.512 | | 1995 | 4.662 | | 1994 | 4.807 | | 1993 | 4.955 | | 1992 | 5.107 | | 1991 | 5.285 | | 1990 | 5.478 | | 1989 | 5.731 | | 1988 | 5.933 | | 1987 | 6.118 | | 1986 | 6.315 | | 1985 | 6.458 | | 1984 | 6.611 | | 1983 | 6.768 | | 1982 | 6.928 | | 1981 | 7.1 | | 1980 | 7.266 | | 1979 | 7.428 | | 1978 | 7.58 | | 1977 | 7.708 | | 1976 | 7.81 | | 1975 | 7.891 | | 1974 | 7.955 | | 1973 | 8.016 | | 1972 | 8.059 | | 1971 | 8.089 | | 1970 | 8.109 | | 1969 | 8.128 | | 1968 | 8.12 | | 1967 | 8.107 | | 1966 | 8.091 | | 1965 | 8.078 | | 1964 | 7.985 | | 1963 | 7.919 | | 1962 | 7.847 | | 1961 | 7.769 | | 1960 | 7.669 |
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