| Year | Value | | 2021 | 1.944 | | 2020 | 1.955 | | 2019 | 1.948 | | 2018 | 1.937 | | 2017 | 1.93 | | 2016 | 1.938 | | 2015 | 1.951 | | 2014 | 1.95 | | 2013 | 1.951 | | 2012 | 1.927 | | 2011 | 1.902 | | 2010 | 1.895 | | 2009 | 1.906 | | 2008 | 1.92 | | 2007 | 1.913 | | 2006 | 1.904 | | 2005 | 1.962 | | 2004 | 2.014 | | 2003 | 2.026 | | 2002 | 2.063 | | 2001 | 2.051 | | 2000 | 2.072 | | 1999 | 2.095 | | 1998 | 2.169 | | 1997 | 2.29 | | 1996 | 2.444 | | 1995 | 2.664 | | 1994 | 2.92 | | 1993 | 3.143 | | 1992 | 3.322 | | 1991 | 3.476 | | 1990 | 3.596 | | 1989 | 3.702 | | 1988 | 3.814 | | 1987 | 3.938 | | 1986 | 4.084 | | 1985 | 4.215 | | 1984 | 4.349 | | 1983 | 4.486 | | 1982 | 4.621 | | 1981 | 4.752 | | 1980 | 4.858 | | 1979 | 4.986 | | 1978 | 5.119 | | 1977 | 5.262 | | 1976 | 5.45 | | 1975 | 5.642 | | 1974 | 5.82 | | 1973 | 6.108 | | 1972 | 6.1 | | 1971 | 6.069 | | 1970 | 6.019 | | 1969 | 6.013 | | 1968 | 6.029 | | 1967 | 5.883 | | 1966 | 5.968 | | 1965 | 6.156 | | 1964 | 6.285 | | 1963 | 6.385 | | 1962 | 6.274 | | 1961 | 5.476 | | 1960 | 6.28 |
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