| Year | Value | | 2021 | 2.086 | | 2020 | 2.114 | | 2019 | 2.148 | | 2018 | 2.174 | | 2017 | 2.221 | | 2016 | 2.238 | | 2015 | 2.251 | | 2014 | 2.252 | | 2013 | 2.24 | | 2012 | 2.216 | | 2011 | 2.191 | | 2010 | 2.105 | | 2009 | 2.046 | | 2008 | 2.011 | | 2007 | 1.989 | | 2006 | 1.977 | | 2005 | 1.981 | | 2004 | 1.976 | | 2003 | 1.979 | | 2002 | 1.969 | | 2001 | 1.997 | | 2000 | 2.049 | | 1999 | 2.099 | | 1998 | 2.219 | | 1997 | 2.359 | | 1996 | 2.505 | | 1995 | 2.673 | | 1994 | 2.856 | | 1993 | 3.017 | | 1992 | 3.175 | | 1991 | 3.324 | | 1990 | 3.471 | | 1989 | 3.585 | | 1988 | 3.851 | | 1987 | 4.1 | | 1986 | 4.358 | | 1985 | 4.516 | | 1984 | 4.669 | | 1983 | 4.786 | | 1982 | 4.91 | | 1981 | 5.05 | | 1980 | 5.105 | | 1979 | 5.294 | | 1978 | 5.511 | | 1977 | 5.772 | | 1976 | 5.936 | | 1975 | 6.026 | | 1974 | 6.111 | | 1973 | 6.185 | | 1972 | 6.264 | | 1971 | 6.359 | | 1970 | 6.517 | | 1969 | 6.685 | | 1968 | 6.835 | | 1967 | 6.955 | | 1966 | 7.052 | | 1965 | 7.1 | | 1964 | 7.015 | | 1963 | 7.009 | | 1962 | 6.993 | | 1961 | 6.97 | | 1960 | 6.942 |
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