| Year | Value | | 2021 | 4.546 | | 2020 | 5.398 | | 2019 | 4.392 | | 2018 | 4.482 | | 2017 | 4.542 | | 2016 | 4.472 | | 2015 | 4.437 | | 2014 | 4.555 | | 2013 | 4.673 | | 2012 | 4.767 | | 2011 | 4.713 | | 2010 | 4.754 | | 2009 | 4.735 | | 2008 | 4.91 | | 2007 | 4.945 | | 2006 | 4.947 | | 2005 | 4.943 | | 2004 | 4.95 | | 2003 | 5.016 | | 2002 | 4.932 | | 2001 | 4.987 | | 2000 | 5.037 | | 1999 | 5.131 | | 1998 | 5.288 | | 1997 | 5.201 | | 1996 | 5.188 | | 1995 | 5.69 | | 1994 | 5.758 | | 1993 | 5.629 | | 1992 | 5.617 | | 1991 | 5.629 | | 1990 | 5.731 | | 1989 | 5.884 | | 1988 | 5.972 | | 1987 | 6.447 | | 1986 | 6.818 | | 1985 | 7.579 | | 1984 | 8.556 | | 1983 | 9.727 | | 1982 | 10.959 | | 1981 | 12.285 | | 1980 | 13.583 | | 1979 | 14.615 | | 1978 | 15.661 | | 1977 | 16.631 | | 1976 | 17.495 | | 1975 | 18.065 | | 1974 | 18.903 | | 1973 | 19.79 | | 1972 | 20.542 | | 1971 | 21.276 | | 1970 | 21.882 | | 1969 | 22.381 | | 1968 | 22.684 | | 1967 | 22.953 | | 1966 | 23.215 | | 1965 | 23.106 | | 1964 | 22.917 | | 1963 | 22.615 | | 1962 | 25.046 | | 1961 | 23.723 | | 1960 | 23.785 |
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