| Year | Value | | 2021 | 1.533 | | 2020 | 1.555 | | 2019 | 1.632 | | 2018 | 1.709 | | 2017 | 1.74 | | 2016 | 1.754 | | 2015 | 1.79 | | 2014 | 1.819 | | 2013 | 1.838 | | 2012 | 1.876 | | 2011 | 1.902 | | 2010 | 1.928 | | 2009 | 1.982 | | 2008 | 2.021 | | 2007 | 2.012 | | 2006 | 2.014 | | 2005 | 2.044 | | 2004 | 2.078 | | 2003 | 2.137 | | 2002 | 2.2 | | 2001 | 2.325 | | 2000 | 2.412 | | 1999 | 2.48 | | 1998 | 2.534 | | 1997 | 2.64 | | 1996 | 2.71 | | 1995 | 2.804 | | 1994 | 2.888 | | 1993 | 2.964 | | 1992 | 3.037 | | 1991 | 3.119 | | 1990 | 3.205 | | 1989 | 3.267 | | 1988 | 3.336 | | 1987 | 3.397 | | 1986 | 3.463 | | 1985 | 3.513 | | 1984 | 3.52 | | 1983 | 3.528 | | 1982 | 3.541 | | 1981 | 3.559 | | 1980 | 3.59 | | 1979 | 3.651 | | 1978 | 3.662 | | 1977 | 3.701 | | 1976 | 3.747 | | 1975 | 3.804 | | 1974 | 3.893 | | 1973 | 3.991 | | 1972 | 4.158 | | 1971 | 4.362 | | 1970 | 4.585 | | 1969 | 4.835 | | 1968 | 5.028 | | 1967 | 5.416 | | 1966 | 5.697 | | 1965 | 5.96 | | 1964 | 6.193 | | 1963 | 6.389 | | 1962 | 6.543 | | 1961 | 6.651 | | 1960 | 6.712 |
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