| Year | Value | | 2021 | 5.168 | | 2020 | 5.086 | | 2019 | 4.968 | | 2018 | 4.857 | | 2017 | 4.735 | | 2016 | 4.581 | | 2015 | 4.451 | | 2014 | 4.429 | | 2013 | 4.305 | | 2012 | 4.231 | | 2011 | 4.188 | | 2010 | 4.114 | | 2009 | 4.048 | | 2008 | 3.955 | | 2007 | 3.844 | | 2006 | 3.731 | | 2005 | 3.563 | | 2004 | 3.4 | | 2003 | 3.23 | | 2002 | 3.218 | | 2001 | 3.54 | | 2000 | 3.38 | | 1999 | 3.287 | | 1998 | 3.166 | | 1997 | 3.075 | | 1996 | 3.047 | | 1995 | 2.953 | | 1994 | 2.785 | | 1993 | 2.702 | | 1992 | 2.656 | | 1991 | 2.602 | | 1987 | 2.5 | | 1986 | 2.3 | | 1985 | 2.2 | | 1984 | 2.2 | | 1983 | 2.1 | | 1982 | 2.1 | | 1981 | 2 | | 1980 | 2 | | 1979 | 1.9 | | 1978 | 1.9 | | 1977 | 1.9 | | 1976 | 1.8 | | 1975 | 1.7 | | 1974 | 1.7 | | 1973 | 1.6 | | 1972 | 1.5 | | 1971 | 1.5 | | 1970 | 1.4 | | 1969 | 1.4 | | 1968 | 1.4 | | 1967 | 1.3 | | 1966 | 1.3 | | 1965 | 1.3 | | 1964 | 1.2 | | 1963 | 1.1 | | 1962 | 1.2 | | 1960 | 1.2 |
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