| Year | Value | | 2021 | 7.426 | | 2020 | 7.221 | | 2019 | 6.346 | | 2018 | 6.327 | | 2017 | 6.139 | | 2016 | 6.107 | | 2015 | 6.121 | | 2014 | 6.12 | | 2013 | 6.037 | | 2012 | 6.074 | | 2011 | 5.805 | | 2010 | 5.962 | | 2009 | 5.612 | | 2008 | 5.557 | | 2007 | 5.754 | | 2006 | 5.352 | | 2005 | 5.301 | | 2004 | 5.293 | | 2003 | 5.247 | | 2002 | 5.035 | | 2001 | 5.157 | | 2000 | 4.959 | | 1999 | 5.246 | | 1998 | 5.258 | | 1997 | 5.138 | | 1996 | 5.337 | | 1995 | 5.352 | | 1994 | 5.217 | | 1993 | 5.316 | | 1992 | 5.201 | | 1991 | 5.313 | | 1990 | 5.625 | | 1989 | 5.538 | | 1988 | 5.459 | | 1987 | 5.327 | | 1986 | 5.54 | | 1985 | 5.732 | | 1984 | 5.943 | | 1983 | 6.044 | | 1982 | 5.77 | | 1981 | 5.823 | | 1980 | 6.212 | | 1979 | 6.422 | | 1978 | 6.323 | | 1977 | 6.497 | | 1976 | 7.158 | | 1975 | 6.806 | | 1974 | 7.374 | | 1973 | 7.764 | | 1972 | 8.524 | | 1971 | 8.298 | | 1970 | 8.365 | | 1969 | 8.679 | | 1968 | 8.718 | | 1967 | 9.128 | | 1966 | 9.647 | | 1965 | 9.755 | | 1964 | 10.094 | | 1963 | 10.982 | | 1962 | 10.577 | | 1961 | 10.748 | | 1960 | 10.992 |
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