| Year | Value | | 2021 | 6.257 | | 2020 | 3.208 | | 2019 | 3.157 | | 2018 | 3.124 | | 2017 | 3.08 | | 2016 | 3.078 | | 2015 | 3.025 | | 2014 | 2.984 | | 2013 | 2.972 | | 2012 | 2.954 | | 2011 | 2.938 | | 2010 | 2.925 | | 2009 | 2.924 | | 2008 | 2.916 | | 2007 | 2.897 | | 2006 | 2.876 | | 2005 | 2.86 | | 2004 | 2.859 | | 2003 | 2.85 | | 2002 | 2.852 | | 2001 | 2.842 | | 2000 | 2.825 | | 1999 | 2.801 | | 1998 | 3.74 | | 1997 | 3.688 | | 1996 | 3.62 | | 1995 | 3.558 | | 1994 | 3.524 | | 1993 | 3.459 | | 1992 | 3.401 | | 1991 | 3.331 | | 1990 | 3.268 | | 1989 | 3.2 | | 1988 | 3.1 | | 1987 | 3 | | 1986 | 2.9 | | 1985 | 2.8 | | 1984 | 2.7 | | 1983 | 2.8 | | 1982 | 2.5 | | 1981 | 2.4 | | 1980 | 2.3 | | 1979 | 2.2 | | 1978 | 2.1 | | 1977 | 2 | | 1976 | 1.9 | | 1975 | 1.9 | | 1974 | 1.8 | | 1973 | 1.7 | | 1972 | 1.6 | | 1971 | 1.5 | | 1969 | 1.6 | | 1968 | 1.6 | | 1967 | 1.5 | | 1966 | 1.5 | | 1965 | 1.5 | | 1964 | 1.4 | | 1963 | 1.4 | | 1962 | 1.4 | | 1961 | 1.3 | | 1960 | 1.3 |
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