| Year | Value | | 2021 | 1.811 | | 2020 | 1.832 | | 2019 | 1.857 | | 2018 | 1.93 | | 2017 | 2.006 | | 2016 | 2.073 | | 2015 | 2.131 | | 2014 | 2.158 | | 2013 | 2.164 | | 2012 | 2.161 | | 2011 | 2.093 | | 2010 | 2.053 | | 2009 | 2.087 | | 2008 | 2.172 | | 2007 | 2.289 | | 2006 | 2.455 | | 2005 | 2.619 | | 2004 | 2.71 | | 2003 | 2.727 | | 2002 | 2.69 | | 2001 | 2.664 | | 2000 | 2.778 | | 1999 | 2.869 | | 1998 | 2.889 | | 1997 | 2.936 | | 1996 | 3.007 | | 1995 | 3.127 | | 1994 | 3.263 | | 1993 | 3.415 | | 1992 | 3.519 | | 1991 | 3.593 | | 1990 | 3.758 | | 1989 | 3.897 | | 1988 | 3.965 | | 1987 | 4.089 | | 1986 | 4.22 | | 1985 | 4.278 | | 1984 | 4.323 | | 1983 | 4.44 | | 1982 | 4.537 | | 1981 | 4.643 | | 1980 | 4.801 | | 1979 | 4.971 | | 1978 | 5.151 | | 1977 | 5.306 | | 1976 | 5.464 | | 1975 | 5.624 | | 1974 | 5.785 | | 1973 | 5.961 | | 1972 | 6.128 | | 1971 | 6.284 | | 1970 | 6.427 | | 1969 | 6.562 | | 1968 | 6.703 | | 1967 | 6.828 | | 1966 | 6.922 | | 1965 | 6.996 | | 1964 | 7.061 | | 1963 | 7.068 | | 1962 | 7.096 | | 1961 | 7.123 | | 1960 | 7.152 |
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