| Year | Value | | 2021 | 1.699 | | 2020 | 1.705 | | 2019 | 1.712 | | 2018 | 1.789 | | 2017 | 1.804 | | 2016 | 1.852 | | 2015 | 1.905 | | 2014 | 1.959 | | 2013 | 1.946 | | 2012 | 2.026 | | 2011 | 2.066 | | 2010 | 2.162 | | 2009 | 2.146 | | 2008 | 2.193 | | 2007 | 2.133 | | 2006 | 2.218 | | 2005 | 2.188 | | 2004 | 2.202 | | 2003 | 2.266 | | 2002 | 2.427 | | 2001 | 2.521 | | 2000 | 2.596 | | 1999 | 2.582 | | 1998 | 2.487 | | 1997 | 2.591 | | 1996 | 2.718 | | 1995 | 2.78 | | 1994 | 2.943 | | 1993 | 3.085 | | 1992 | 3.15 | | 1991 | 3.258 | | 1990 | 3.437 | | 1989 | 3.488 | | 1988 | 3.751 | | 1987 | 3.62 | | 1986 | 3.714 | | 1985 | 3.863 | | 1984 | 3.834 | | 1983 | 3.743 | | 1982 | 3.796 | | 1981 | 3.935 | | 1980 | 3.935 | | 1979 | 3.941 | | 1978 | 4.043 | | 1977 | 4.366 | | 1976 | 4.517 | | 1975 | 4.707 | | 1974 | 4.909 | | 1973 | 5.112 | | 1972 | 5.293 | | 1971 | 5.464 | | 1970 | 5.624 | | 1969 | 5.738 | | 1968 | 5.793 | | 1967 | 5.827 | | 1966 | 5.856 | | 1965 | 5.877 | | 1964 | 5.88 | | 1963 | 5.884 | | 1962 | 5.889 | | 1961 | 5.887 | | 1960 | 5.888 |
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