| Year | Value | | 2021 | 1.39 | | 2020 | 1.4 | | 2019 | 1.414 | | 2018 | 1.44 | | 2017 | 1.491 | | 2016 | 1.551 | | 2015 | 1.613 | | 2014 | 1.674 | | 2013 | 1.697 | | 2012 | 1.692 | | 2011 | 1.671 | | 2010 | 1.656 | | 2009 | 1.647 | | 2008 | 1.648 | | 2007 | 1.671 | | 2006 | 1.724 | | 2005 | 1.803 | | 2004 | 1.89 | | 2003 | 1.978 | | 2002 | 2.036 | | 2001 | 2.15 | | 2000 | 2.231 | | 1999 | 2.318 | | 1998 | 2.397 | | 1997 | 2.497 | | 1996 | 2.615 | | 1995 | 2.685 | | 1994 | 2.707 | | 1993 | 2.707 | | 1992 | 2.767 | | 1991 | 2.826 | | 1990 | 2.9 | | 1989 | 2.942 | | 1988 | 3.004 | | 1987 | 3.061 | | 1986 | 3.116 | | 1985 | 3.205 | | 1984 | 3.296 | | 1983 | 3.361 | | 1982 | 3.452 | | 1981 | 3.509 | | 1980 | 3.614 | | 1979 | 3.763 | | 1978 | 3.946 | | 1977 | 4.136 | | 1976 | 4.34 | | 1975 | 4.524 | | 1974 | 4.642 | | 1973 | 4.764 | | 1972 | 4.898 | | 1971 | 5.015 | | 1970 | 5.111 | | 1969 | 5.258 | | 1968 | 5.338 | | 1967 | 5.404 | | 1966 | 5.465 | | 1965 | 5.623 | | 1964 | 5.849 | | 1963 | 6.047 | | 1962 | 6.207 | | 1961 | 6.353 | | 1960 | 6.455 |
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