| Year | Value | | 2021 | 13.5 | | 2020 | 12.2 | | 2019 | 11.7 | | 2018 | 11.2 | | 2017 | 11.6 | | 2016 | 11 | | 2015 | 11.2 | | 2014 | 10.4 | | 2013 | 10.2 | | 2012 | 10.6 | | 2011 | 10 | | 2010 | 9.8 | | 2009 | 9.8 | | 2008 | 9.7 | | 2007 | 9.9 | | 2006 | 9.6 | | 2005 | 9.6 | | 2004 | 9.6 | | 2003 | 9.7 | | 2002 | 9.5 | | 2001 | 9.4 | | 2000 | 9.7 | | 1999 | 9.6 | | 1998 | 9.6 | | 1997 | 9.4 | | 1996 | 9.5 | | 1995 | 9.5 | | 1994 | 9.3 | | 1993 | 9.3 | | 1992 | 9.4 | | 1991 | 9.3 | | 1990 | 9.2 | | 1989 | 9.2 | | 1988 | 9.2 | | 1987 | 9.6 | | 1986 | 9.2 | | 1985 | 9.4 | | 1984 | 8.9 | | 1983 | 9.2 | | 1982 | 8.8 | | 1981 | 8.9 | | 1980 | 9.1 | | 1979 | 8.6 | | 1978 | 8.7 | | 1977 | 9 | | 1976 | 8.9 | | 1975 | 8.9 | | 1974 | 8.5 | | 1973 | 8.7 | | 1972 | 8.6 | | 1971 | 8.4 | | 1970 | 8.4 | | 1969 | 8.2 | | 1968 | 8.4 | | 1967 | 8.3 | | 1966 | 7.9 | | 1965 | 7.9 | | 1964 | 8.2 | | 1963 | 7.9 | | 1962 | 7.9 | | 1961 | 7.6 | | 1960 | 7.3 |
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