| Year | Value | | 2021 | 2.344 | | 2020 | 2.381 | | 2019 | 2.402 | | 2018 | 2.435 | | 2017 | 2.473 | | 2016 | 2.512 | | 2015 | 2.55 | | 2014 | 2.589 | | 2013 | 2.62 | | 2012 | 2.664 | | 2011 | 2.701 | | 2010 | 2.774 | | 2009 | 2.871 | | 2008 | 2.969 | | 2007 | 3.084 | | 2006 | 3.145 | | 2005 | 3.244 | | 2004 | 3.345 | | 2003 | 3.442 | | 2002 | 3.555 | | 2001 | 3.652 | | 2000 | 3.772 | | 1999 | 3.922 | | 1998 | 4.139 | | 1997 | 4.391 | | 1996 | 4.594 | | 1995 | 4.824 | | 1994 | 5.052 | | 1993 | 5.274 | | 1992 | 5.447 | | 1991 | 5.565 | | 1990 | 5.644 | | 1989 | 5.723 | | 1988 | 5.944 | | 1987 | 6.097 | | 1986 | 6.226 | | 1985 | 6.313 | | 1984 | 6.338 | | 1983 | 6.296 | | 1982 | 6.193 | | 1981 | 6.042 | | 1980 | 5.774 | | 1979 | 4.556 | | 1978 | 3.338 | | 1977 | 2.716 | | 1976 | 3.409 | | 1975 | 4.103 | | 1974 | 4.797 | | 1973 | 5.559 | | 1972 | 5.991 | | 1971 | 5.866 | | 1970 | 5.72 | | 1969 | 5.559 | | 1968 | 5.648 | | 1967 | 5.712 | | 1966 | 5.769 | | 1965 | 5.823 | | 1964 | 5.862 | | 1963 | 5.852 | | 1962 | 5.996 | | 1961 | 6.122 | | 1960 | 6.251 |
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