| Year | Value | | 2021 | 2.837 | | 2020 | 2.9 | | 2019 | 2.937 | | 2018 | 2.923 | | 2017 | 2.891 | | 2016 | 2.957 | | 2015 | 3.01 | | 2014 | 2.996 | | 2013 | 2.897 | | 2012 | 2.739 | | 2011 | 2.602 | | 2010 | 2.513 | | 2009 | 2.533 | | 2008 | 2.446 | | 2007 | 2.297 | | 2006 | 2.13 | | 2005 | 2.028 | | 2004 | 2.017 | | 2003 | 2.051 | | 2002 | 2.13 | | 2001 | 2.195 | | 2000 | 2.255 | | 1999 | 2.302 | | 1998 | 2.363 | | 1997 | 2.469 | | 1996 | 2.618 | | 1995 | 2.789 | | 1994 | 2.947 | | 1993 | 3.163 | | 1992 | 3.524 | | 1991 | 3.905 | | 1990 | 4.232 | | 1989 | 4.445 | | 1988 | 4.682 | | 1987 | 4.85 | | 1986 | 5.039 | | 1985 | 5.271 | | 1984 | 5.468 | | 1983 | 5.653 | | 1982 | 5.845 | | 1981 | 6.033 | | 1980 | 6.26 | | 1979 | 6.469 | | 1978 | 6.669 | | 1977 | 6.823 | | 1976 | 6.967 | | 1975 | 7.134 | | 1974 | 7.268 | | 1973 | 7.354 | | 1972 | 7.403 | | 1971 | 7.433 | | 1970 | 7.464 | | 1969 | 7.487 | | 1968 | 7.517 | | 1967 | 7.529 | | 1966 | 7.545 | | 1965 | 7.482 | | 1964 | 7.451 | | 1963 | 7.383 | | 1962 | 7.238 | | 1961 | 7.007 | | 1960 | 6.827 |
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