| Year | Value | | 2021 | 15.5 | | 2020 | 12.4 | | 2019 | 9.8 | | 2018 | 9.5 | | 2017 | 9.8 | | 2016 | 9.853364793075 | | 2015 | 9.9 | | 2014 | 9.5 | | 2013 | 9.3 | | 2012 | 9.8 | | 2011 | 9.5 | | 2010 | 9.3 | | 2009 | 9.3 | | 2008 | 9.3 | | 2007 | 9.6 | | 2006 | 9.1 | | 2005 | 9 | | 2004 | 8.8 | | 2003 | 8.9 | | 2002 | 8.9 | | 2001 | 8.3 | | 2000 | 8.5 | | 1999 | 8.3 | | 1998 | 8.4 | | 1997 | 8.3 | | 1996 | 8.478 | | 1995 | 8.573 | | 1994 | 8.446 | | 1993 | 7.972 | | 1992 | 8.12 | | 1991 | 7.497 | | 1990 | 7.606 | | 1989 | 7.436 | | 1988 | 7.417 | | 1987 | 7.505 | | 1986 | 7.376 | | 1985 | 7.435 | | 1984 | 7.324 | | 1983 | 7.457 | | 1982 | 7.057 | | 1981 | 6.978 | | 1980 | 7.101 | | 1979 | 6.795 | | 1978 | 6.781 | | 1977 | 7.055 | | 1976 | 7.295 | | 1975 | 7.528 | | 1974 | 7.587 | | 1973 | 7.787 | | 1972 | 7.871 | | 1971 | 8.23 | | 1970 | 8.599 | | 1969 | 8.914 | | 1968 | 8.971 | | 1967 | 9.318 | | 1966 | 9.406 | | 1965 | 10.4 | | 1964 | 10.974 | | 1963 | 12.262 | | 1962 | 11.985 | | 1961 | 11.946 | | 1960 | 12.966 |
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