| Year | Value | | 2021 | 5 | | 2020 | 5.1 | | 2019 | 4.6 | | 2018 | 4.7 | | 2017 | 5 | | 2016 | 4.9 | | 2015 | 4.9 | | 2014 | 4.9 | | 2013 | 4.8 | | 2012 | 4.9 | | 2011 | 4.9 | | 2010 | 4.8 | | 2009 | 4.8 | | 2008 | 5.1 | | 2007 | 5.1 | | 2006 | 5.3 | | 2005 | 5.4 | | 2004 | 5 | | 2003 | 5.3 | | 2002 | 5.4 | | 2001 | 5.3 | | 2000 | 5.5 | | 1999 | 5.4 | | 1998 | 5.9 | | 1997 | 5.8 | | 1996 | 6.2 | | 1995 | 6.4 | | 1994 | 6.6 | | 1993 | 6.6 | | 1992 | 6.5 | | 1991 | 6.2 | | 1990 | 6.1 | | 1989 | 6.3 | | 1988 | 6.9 | | 1987 | 7 | | 1986 | 7.1 | | 1985 | 7.2 | | 1984 | 7.5 | | 1983 | 7.5 | | 1982 | 7.4 | | 1981 | 7.2 | | 1980 | 7.5 | | 1979 | 7 | | 1978 | 9.588 | | 1977 | 9.795 | | 1976 | 9.765 | | 1975 | 10.004 | | 1974 | 10.153 | | 1973 | 10.394 | | 1972 | 10.468 | | 1971 | 10.69 | | 1970 | 10.979 | | 1969 | 11.184 | | 1968 | 11.624 | | 1967 | 12.1 | | 1966 | 12.557 | | 1965 | 12.995 | | 1964 | 13.446 | | 1963 | 13.948 | | 1962 | 14.427 | | 1961 | 14.727 | | 1960 | 14.995 |
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