| Year | Value | | 2021 | 14.8 | | 2020 | 11.7 | | 2019 | 10.6 | | 2018 | 10.5 | | 2017 | 10.5 | | 2016 | 10.4 | | 2015 | 10.2 | | 2014 | 9.7 | | 2013 | 9.5 | | 2012 | 9.5 | | 2011 | 9.4 | | 2010 | 9.1 | | 2009 | 9.5 | | 2008 | 9.3 | | 2007 | 9.7 | | 2006 | 9.7 | | 2005 | 9.5 | | 2004 | 9.3 | | 2003 | 9.3 | | 2002 | 9 | | 2001 | 8.9 | | 2000 | 9 | | 1999 | 8.9 | | 1998 | 8.7 | | 1997 | 8.5 | | 1996 | 8.2 | | 1995 | 8 | | 1994 | 8.723 | | 1993 | 7.702 | | 1992 | 7.728 | | 1991 | 6.948 | | 1990 | 6.772 | | 1989 | 6.617 | | 1988 | 6.094 | | 1987 | 6.491 | | 1986 | 6.727 | | 1985 | 6.867 | | 1984 | 7.264 | | 1983 | 8.143 | | 1982 | 8.024 | | 1981 | 7.865 | | 1980 | 7.568 | | 1979 | 8.008 | | 1978 | 7.617 | | 1977 | 7.382 | | 1976 | 7.162 | | 1975 | 7.24 | | 1974 | 7.096 | | 1973 | 7.313 | | 1972 | 7.323 | | 1971 | 7.637 | | 1970 | 7.787 | | 1969 | 7.74 | | 1968 | 7.539 | | 1967 | 7.655 | | 1966 | 7.626 | | 1965 | 8.024 | | 1964 | 8.42 | | 1963 | 8.61 | | 1962 | 9.11 | | 1961 | 9.397 | | 1960 | 9.677 |
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