| Year | Value | | 2021 | 1.633 | | 2020 | 1.628 | | 2019 | 1.623 | | 2018 | 1.619 | | 2017 | 1.615 | | 2016 | 1.613 | | 2015 | 1.614 | | 2014 | 1.62 | | 2013 | 1.63 | | 2012 | 1.645 | | 2011 | 1.665 | | 2010 | 1.687 | | 2009 | 1.711 | | 2008 | 1.735 | | 2007 | 1.757 | | 2006 | 1.775 | | 2005 | 1.788 | | 2004 | 1.795 | | 2003 | 1.797 | | 2002 | 1.793 | | 2001 | 1.785 | | 2000 | 1.775 | | 1999 | 1.762 | | 1998 | 1.75 | | 1997 | 1.74 | | 1996 | 1.732 | | 1995 | 1.727 | | 1994 | 1.725 | | 1993 | 1.725 | | 1992 | 1.728 | | 1991 | 1.733 | | 1990 | 1.74 | | 1989 | 1.75 | | 1988 | 1.765 | | 1987 | 1.782 | | 1986 | 1.805 | | 1985 | 1.83 | | 1984 | 1.858 | | 1983 | 1.889 | | 1982 | 1.922 | | 1981 | 1.96 | | 1980 | 2.004 | | 1979 | 2.057 | | 1978 | 2.121 | | 1977 | 2.198 | | 1976 | 2.288 | | 1975 | 2.393 | | 1974 | 2.513 | | 1973 | 2.648 | | 1972 | 2.794 | | 1971 | 2.949 | | 1970 | 3.113 | | 1969 | 3.284 | | 1968 | 3.46 | | 1967 | 3.637 | | 1966 | 3.807 | | 1965 | 3.963 | | 1964 | 4.094 | | 1963 | 4.196 | | 1962 | 4.269 | | 1961 | 4.313 | | 1960 | 4.333 |
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