| Year | Value | | 2021 | 1.41 | | 2020 | 1.44 | | 2019 | 1.4 | | 2018 | 1.41 | | 2017 | 1.44 | | 2016 | 1.4 | | 2015 | 1.36 | | 2014 | 1.43 | | 2013 | 1.44 | | 2012 | 1.54 | | 2011 | 1.55 | | 2010 | 1.57 | | 2009 | 1.59 | | 2008 | 1.67 | | 2007 | 1.74 | | 2006 | 1.77 | | 2005 | 1.88 | | 2004 | 1.92 | | 2003 | 1.87 | | 2002 | 1.94 | | 2001 | 1.91 | | 2000 | 1.99 | | 1999 | 2.05 | | 1998 | 1.97 | | 1997 | 2.04 | | 1996 | 2.12 | | 1995 | 2.14 | | 1994 | 2.25 | | 1993 | 2.21 | | 1992 | 2.37 | | 1991 | 2.3 | | 1990 | 2.32 | | 1989 | 2.23 | | 1988 | 2.14 | | 1987 | 2.05 | | 1986 | 1.99 | | 1985 | 2.02 | | 1984 | 2.16 | | 1983 | 2.394 | | 1982 | 2.513 | | 1981 | 2.648 | | 1980 | 2.782 | | 1979 | 2.894 | | 1978 | 2.99 | | 1977 | 3.061 | | 1976 | 3.13 | | 1975 | 3.196 | | 1974 | 3.341 | | 1973 | 3.444 | | 1972 | 3.575 | | 1971 | 3.741 | | 1970 | 3.952 | | 1969 | 4.213 | | 1968 | 4.516 | | 1967 | 4.847 | | 1966 | 5.187 | | 1965 | 5.51 | | 1964 | 5.785 | | 1963 | 5.993 | | 1962 | 6.124 | | 1961 | 6.178 | | 1960 | 6.167 |
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