| Year | Value | | 2021 | 2.091 | | 2020 | 2.103 | | 2019 | 2.13 | | 2018 | 2.148 | | 2017 | 2.173 | | 2016 | 2.183 | | 2015 | 2.178 | | 2014 | 2.182 | | 2013 | 2.172 | | 2012 | 2.169 | | 2011 | 2.162 | | 2010 | 2.126 | | 2009 | 2.085 | | 2008 | 2.081 | | 2007 | 2.11 | | 2006 | 2.157 | | 2005 | 2.203 | | 2004 | 2.27 | | 2003 | 2.345 | | 2002 | 2.41 | | 2001 | 2.462 | | 2000 | 2.499 | | 1999 | 2.55 | | 1998 | 2.6 | | 1997 | 2.659 | | 1996 | 2.736 | | 1995 | 2.781 | | 1994 | 2.868 | | 1993 | 2.97 | | 1992 | 3.081 | | 1991 | 3.19 | | 1990 | 3.295 | | 1989 | 3.391 | | 1988 | 3.4 | | 1987 | 3.431 | | 1986 | 3.501 | | 1985 | 3.587 | | 1984 | 3.704 | | 1983 | 3.789 | | 1982 | 3.885 | | 1981 | 3.984 | | 1980 | 4.032 | | 1979 | 4.091 | | 1978 | 4.196 | | 1977 | 4.308 | | 1976 | 4.418 | | 1975 | 4.559 | | 1974 | 4.685 | | 1973 | 4.805 | | 1972 | 4.933 | | 1971 | 5.042 | | 1970 | 5.168 | | 1969 | 5.279 | | 1968 | 5.382 | | 1967 | 5.485 | | 1966 | 5.57 | | 1965 | 5.647 | | 1964 | 5.724 | | 1963 | 5.779 | | 1962 | 5.797 | | 1961 | 5.809 | | 1960 | 5.818 |
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