| Year | Value | | 2021 | 6.437 | | 2020 | 5.567 | | 2019 | 4.872 | | 2018 | 4.715 | | 2017 | 4.692 | | 2016 | 4.726 | | 2015 | 4.842 | | 2014 | 4.905 | | 2013 | 4.911 | | 2012 | 4.892 | | 2011 | 4.978 | | 2010 | 5.108 | | 2009 | 5.127 | | 2008 | 5.041 | | 2007 | 4.838 | | 2006 | 4.911 | | 2005 | 4.97 | | 2004 | 5.007 | | 2003 | 5.459 | | 2002 | 5.1 | | 2001 | 5.168 | | 2000 | 5.15 | | 1999 | 5.18 | | 1998 | 5.311 | | 1997 | 5.357 | | 1996 | 5.415 | | 1995 | 5.514 | | 1994 | 5.661 | | 1993 | 5.821 | | 1992 | 6.015 | | 1991 | 6.207 | | 1990 | 7.176 | | 1989 | 6.686 | | 1988 | 7.668 | | 1987 | 8.071 | | 1986 | 8.373 | | 1985 | 8.78 | | 1984 | 9.219 | | 1983 | 10.42 | | 1982 | 10.995 | | 1981 | 11.621 | | 1980 | 10.95 | | 1979 | 11.047 | | 1978 | 12.398 | | 1977 | 12.072 | | 1976 | 12.537 | | 1975 | 13.039 | | 1974 | 13.594 | | 1973 | 14.214 | | 1972 | 15.147 | | 1971 | 15.706 | | 1970 | 16.478 | | 1969 | 17.154 | | 1968 | 18.141 | | 1967 | 18.373 | | 1966 | 18.938 | | 1965 | 19.593 | | 1964 | 20.209 | | 1963 | 20.887 | | 1962 | 22.157 | | 1961 | 22.268 | | 1960 | 23.053 |
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