| Year | Value | | 2021 | 7.735 | | 2020 | 6.606 | | 2019 | 5.411 | | 2018 | 5.311 | | 2017 | 5.249 | | 2016 | 5.213 | | 2015 | 5.168 | | 2014 | 5.106 | | 2013 | 5.041 | | 2012 | 4.982 | | 2011 | 4.945 | | 2010 | 4.924 | | 2009 | 4.912 | | 2008 | 4.947 | | 2007 | 4.993 | | 2006 | 5.016 | | 2005 | 5.072 | | 2004 | 5.134 | | 2003 | 5.179 | | 2002 | 5.26 | | 2001 | 5.35 | | 2000 | 5.351 | | 1999 | 5.432 | | 1998 | 5.442 | | 1997 | 5.476 | | 1996 | 5.536 | | 1995 | 5.58 | | 1994 | 5.653 | | 1993 | 5.71 | | 1992 | 5.78 | | 1991 | 5.818 | | 1990 | 5.834 | | 1989 | 5.826 | | 1988 | 5.83 | | 1987 | 5.829 | | 1986 | 5.788 | | 1985 | 6.532 | | 1984 | 5.832 | | 1983 | 5.897 | | 1982 | 5.96 | | 1981 | 6.066 | | 1980 | 6.173 | | 1979 | 6.343 | | 1978 | 6.484 | | 1977 | 6.676 | | 1976 | 6.894 | | 1975 | 7.127 | | 1974 | 7.404 | | 1973 | 7.685 | | 1972 | 7.956 | | 1971 | 8.263 | | 1970 | 8.579 | | 1969 | 8.853 | | 1968 | 9.139 | | 1967 | 9.428 | | 1966 | 9.711 | | 1965 | 10.025 | | 1964 | 10.356 | | 1963 | 10.701 | | 1962 | 11.096 | | 1961 | 11.477 | | 1960 | 11.873 |
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