| Year | Value | | 2021 | 2.151 | | 2020 | 2.174 | | 2019 | 2.2 | | 2018 | 2.213 | | 2017 | 2.234 | | 2016 | 2.249 | | 2015 | 2.248 | | 2014 | 2.24 | | 2013 | 2.256 | | 2012 | 2.274 | | 2011 | 2.312 | | 2010 | 2.346 | | 2009 | 2.391 | | 2008 | 2.428 | | 2007 | 2.479 | | 2006 | 2.504 | | 2005 | 2.552 | | 2004 | 2.621 | | 2003 | 2.704 | | 2002 | 2.782 | | 2001 | 2.783 | | 2000 | 2.785 | | 1999 | 2.825 | | 1998 | 2.889 | | 1997 | 2.963 | | 1996 | 3.043 | | 1995 | 3.11 | | 1994 | 3.192 | | 1993 | 3.282 | | 1992 | 3.363 | | 1991 | 3.451 | | 1990 | 3.543 | | 1989 | 3.637 | | 1988 | 3.711 | | 1987 | 3.889 | | 1986 | 4.088 | | 1985 | 4.297 | | 1984 | 4.499 | | 1983 | 4.615 | | 1982 | 4.668 | | 1981 | 4.726 | | 1980 | 4.828 | | 1979 | 4.938 | | 1978 | 5.024 | | 1977 | 5.118 | | 1976 | 5.209 | | 1975 | 5.291 | | 1974 | 5.382 | | 1973 | 5.491 | | 1972 | 5.591 | | 1971 | 5.682 | | 1970 | 5.752 | | 1969 | 5.812 | | 1968 | 5.874 | | 1967 | 5.923 | | 1966 | 5.956 | | 1965 | 5.994 | | 1964 | 5.991 | | 1963 | 5.991 | | 1962 | 5.994 | | 1961 | 5.982 | | 1960 | 5.983 |
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