| Year | Value | | 2021 | 3 | | 2020 | 2.9 | | 2019 | 3.01 | | 2018 | 3.09 | | 2017 | 3.11 | | 2016 | 3.11 | | 2015 | 3.09 | | 2014 | 3.08 | | 2013 | 3.03 | | 2012 | 3.05 | | 2011 | 2.98 | | 2010 | 3.03 | | 2009 | 2.96 | | 2008 | 2.96 | | 2007 | 2.9 | | 2006 | 2.88 | | 2005 | 2.84 | | 2004 | 2.9 | | 2003 | 2.95 | | 2002 | 2.89 | | 2001 | 2.92 | | 2000 | 2.95 | | 1999 | 2.965 | | 1998 | 2.98 | | 1997 | 2.94 | | 1996 | 2.9 | | 1995 | 2.9 | | 1994 | 2.9 | | 1993 | 2.8 | | 1992 | 2.7 | | 1991 | 2.762 | | 1990 | 2.824 | | 1989 | 2.886 | | 1988 | 2.948 | | 1987 | 3.01 | | 1986 | 3.034 | | 1985 | 3.058 | | 1984 | 3.082 | | 1983 | 3.106 | | 1982 | 3.13 | | 1981 | 3.186 | | 1980 | 3.242 | | 1979 | 3.298 | | 1978 | 3.354 | | 1977 | 3.41 | | 1976 | 3.482 | | 1975 | 3.554 | | 1974 | 3.626 | | 1973 | 3.698 | | 1972 | 3.77 | | 1971 | 3.774 | | 1970 | 3.778 | | 1969 | 3.782 | | 1968 | 3.786 | | 1967 | 3.79 | | 1966 | 3.802 | | 1965 | 3.814 | | 1964 | 3.826 | | 1963 | 3.838 | | 1962 | 3.85 | | 1961 | 3.858 | | 1960 | 3.866 |
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