| Year | Value | | 2021 | 3.171 | | 2020 | 3.041 | | 2019 | 2.942 | | 2018 | 2.834 | | 2017 | 2.814 | | 2016 | 2.783 | | 2015 | 2.773 | | 2014 | 2.742 | | 2013 | 2.706 | | 2012 | 2.697 | | 2011 | 2.691 | | 2010 | 2.656 | | 2009 | 2.653 | | 2008 | 2.571 | | 2007 | 2.492 | | 2006 | 2.46 | | 2005 | 2.408 | | 2004 | 2.331 | | 2003 | 2.201 | | 2002 | 2.114 | | 2001 | 2.043 | | 2000 | 1.994 | | 1999 | 1.913 | | 1998 | 1.899 | | 1997 | 1.855 | | 1996 | 1.798 | | 1995 | 1.757 | | 1994 | 1.698 | | 1993 | 1.684 | | 1992 | 1.646 | | 1991 | 1.633 | | 1990 | 1.618 | | 1979 | 1.3 | | 1978 | 1.2 | | 1977 | 1.2 | | 1976 | 1.2 | | 1975 | 1.1 | | 1974 | 1.1 | | 1973 | 1.1 | | 1972 | 1 | | 1971 | 1 | | 1970 | 0.9 | | 1969 | 0.9 | | 1968 | 0.9 | | 1967 | 0.9 | | 1966 | 0.9 | | 1965 | 0.9 | | 1964 | 0.9 | | 1963 | 0.9 | | 1962 | 0.9 | | 1961 | 0.9 | | 1960 | 0.8 |
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