| Year | Value | | 2021 | 8.1 | | 2020 | 7 | | 2019 | 8 | | 2018 | 7.7 | | 2017 | 8.9 | | 2016 | 7.7 | | 2015 | 7.7 | | 2014 | 8.1 | | 2013 | 7.6 | | 2012 | 8.5 | | 2011 | 8 | | 2010 | 7.2 | | 2009 | 7.9 | | 2008 | 7.9 | | 2007 | 7.9 | | 2006 | 8.7 | | 2005 | 8.6 | | 2004 | 7.9 | | 2003 | 8.5 | | 2002 | 8.3 | | 2001 | 7.7 | | 2000 | 7.7 | | 1999 | 8.9 | | 1998 | 8.5 | | 1997 | 8.6 | | 1996 | 9.1 | | 1995 | 8.4 | | 1994 | 8 | | 1993 | 8.4 | | 1992 | 8.5 | | 1991 | 8.4 | | 1990 | 7.5 | | 1989 | 7.8 | | 1988 | 9 | | 1987 | 8 | | 1986 | 8.1 | | 1985 | 7.5 | | 1984 | 7.8 | | 1983 | 8.1 | | 1982 | 7.1 | | 1981 | 6.7 | | 1980 | 7.1 | | 1979 | 6.3 | | 1978 | 7.1 | | 1977 | 6.5 | | 1976 | 7 | | 1975 | 6.9 | | 1974 | 7.6 | | 1973 | 7.7 | | 1972 | 6.9 | | 1971 | 8 | | 1970 | 7.4 |
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