| Year | Value | | 2021 | 6.365 | | 2020 | 5.961 | | 2019 | 5.865 | | 2018 | 5.787 | | 2017 | 5.746 | | 2016 | 5.662 | | 2015 | 5.649 | | 2014 | 5.544 | | 2013 | 5.565 | | 2012 | 5.585 | | 2011 | 5.624 | | 2010 | 5.7 | | 2009 | 5.727 | | 2008 | 5.813 | | 2007 | 5.914 | | 2006 | 6.077 | | 2005 | 6.015 | | 2004 | 6.075 | | 2003 | 6.142 | | 2002 | 6.072 | | 2001 | 6.305 | | 2000 | 6.587 | | 1999 | 6.734 | | 1998 | 6.96 | | 1997 | 7.104 | | 1996 | 7.257 | | 1995 | 7.416 | | 1994 | 7.533 | | 1993 | 7.666 | | 1992 | 7.745 | | 1991 | 7.753 | | 1990 | 7.666 | | 1989 | 7.608 | | 1988 | 7.461 | | 1987 | 7.327 | | 1986 | 7.196 | | 1985 | 7.052 | | 1984 | 6.92 | | 1983 | 6.77 | | 1982 | 6.584 | | 1981 | 6.459 | | 1980 | 6.358 | | 1979 | 6.254 | | 1978 | 6.129 | | 1977 | 6.091 | | 1976 | 5.959 | | 1975 | 5.908 | | 1974 | 5.8 | | 1973 | 5.774 | | 1972 | 5.725 | | 1971 | 5.701 | | 1970 | 5.844 | | 1969 | 6.025 | | 1968 | 6.365 | | 1967 | 6.705 | | 1966 | 7.108 | | 1965 | 7.539 | | 1964 | 7.994 | | 1963 | 8.4 | | 1962 | 8.788 | | 1961 | 9.154 | | 1960 | 9.553 |
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