| Year | Value | | 2021 | 1.885 | | 2020 | 1.911 | | 2019 | 1.994 | | 2018 | 2.039 | | 2017 | 2.168 | | 2016 | 2.241 | | 2015 | 2.301 | | 2014 | 2.312 | | 2013 | 2.322 | | 2012 | 2.33 | | 2011 | 2.338 | | 2010 | 2.346 | | 2009 | 2.344 | | 2008 | 2.341 | | 2007 | 2.37 | | 2006 | 2.4 | | 2005 | 2.426 | | 2004 | 2.43 | | 2003 | 2.451 | | 2002 | 2.493 | | 2001 | 2.531 | | 2000 | 2.591 | | 1999 | 2.643 | | 1998 | 2.662 | | 1997 | 2.706 | | 1996 | 2.781 | | 1995 | 2.864 | | 1994 | 2.935 | | 1993 | 2.966 | | 1992 | 2.987 | | 1991 | 3.012 | | 1990 | 3.034 | | 1989 | 3.022 | | 1988 | 3.042 | | 1987 | 3.059 | | 1986 | 3.075 | | 1985 | 3.095 | | 1984 | 3.123 | | 1983 | 3.159 | | 1982 | 3.204 | | 1981 | 3.253 | | 1980 | 3.301 | | 1979 | 3.339 | | 1978 | 3.359 | | 1977 | 3.359 | | 1976 | 3.339 | | 1975 | 3.301 | | 1974 | 3.251 | | 1973 | 3.198 | | 1972 | 3.146 | | 1971 | 3.108 | | 1970 | 3.088 | | 1969 | 3.052 | | 1968 | 3.027 | | 1967 | 3.039 | | 1966 | 3.048 | | 1965 | 3.062 | | 1964 | 3.08 | | 1963 | 3.101 | | 1962 | 3.106 | | 1961 | 3.069 | | 1960 | 3.075 |
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