| Year | Value | | 2021 | 3.491 | | 2020 | 3.545 | | 2019 | 3.599 | | 2018 | 3.659 | | 2017 | 3.706 | | 2016 | 3.771 | | 2015 | 3.849 | | 2014 | 3.962 | | 2013 | 4.064 | | 2012 | 4.096 | | 2011 | 4.096 | | 2010 | 4.025 | | 2009 | 3.945 | | 2008 | 3.774 | | 2007 | 3.652 | | 2006 | 3.62 | | 2005 | 3.665 | | 2004 | 3.738 | | 2003 | 3.821 | | 2002 | 3.887 | | 2001 | 3.954 | | 2000 | 3.974 | | 1999 | 4.009 | | 1998 | 4.032 | | 1997 | 4.063 | | 1996 | 4.109 | | 1995 | 4.154 | | 1994 | 4.27 | | 1993 | 4.386 | | 1992 | 4.567 | | 1991 | 4.714 | | 1990 | 4.873 | | 1989 | 5.081 | | 1988 | 5.288 | | 1987 | 5.504 | | 1986 | 5.697 | | 1985 | 5.895 | | 1984 | 6.083 | | 1983 | 6.246 | | 1982 | 6.382 | | 1981 | 6.511 | | 1980 | 6.614 | | 1979 | 6.703 | | 1978 | 6.774 | | 1977 | 6.845 | | 1976 | 6.91 | | 1975 | 6.977 | | 1974 | 7.043 | | 1973 | 7.07 | | 1972 | 7.062 | | 1971 | 7.059 | | 1970 | 7.094 | | 1969 | 7.139 | | 1968 | 7.193 | | 1967 | 7.215 | | 1966 | 7.24 | | 1965 | 7.258 | | 1964 | 7.261 | | 1963 | 7.253 | | 1962 | 7.255 | | 1961 | 7.233 | | 1960 | 7.22 |
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