| Year | Value | | 2021 | 4.443 | | 2020 | 4.389 | | 2019 | 4.35 | | 2018 | 4.339 | | 2017 | 4.298 | | 2016 | 4.251 | | 2015 | 4.197 | | 2014 | 4.126 | | 2013 | 4.041 | | 2012 | 3.916 | | 2011 | 3.834 | | 2010 | 3.81 | | 2009 | 3.837 | | 2008 | 3.822 | | 2007 | 3.789 | | 2006 | 3.792 | | 2005 | 3.741 | | 2004 | 3.697 | | 2003 | 3.659 | | 2002 | 3.562 | | 2001 | 3.515 | | 2000 | 3.511 | | 1999 | 3.364 | | 1998 | 3.331 | | 1997 | 3.266 | | 1996 | 3.213 | | 1995 | 3.165 | | 1994 | 3.116 | | 1993 | 3.062 | | 1992 | 3.013 | | 1991 | 3.031 | | 1990 | 2.984 | | 1989 | 2.9 | | 1988 | 3 | | 1987 | 2.9 | | 1986 | 2.8 | | 1985 | 2.7 | | 1984 | 2.6 | | 1983 | 2.6 | | 1982 | 2.6 | | 1981 | 2.6 | | 1980 | 2.5 | | 1979 | 2.4 | | 1978 | 2.2 | | 1977 | 2.1 | | 1976 | 2 | | 1975 | 1.9 | | 1974 | 1.8 | | 1973 | 1.7 | | 1972 | 1.6 | | 1971 | 1.6 | | 1970 | 1.5 | | 1969 | 1.5 | | 1968 | 1.4 | | 1967 | 1.4 | | 1966 | 1.4 | | 1965 | 1.4 | | 1964 | 1.4 | | 1963 | 1.4 | | 1962 | 1.4 | | 1961 | 1.4 | | 1960 | 1.4 |
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