| Year | Value | | 2021 | 7.337 | | 2020 | 7.178 | | 2019 | 6.304 | | 2018 | 6.21 | | 2017 | 6.127 | | 2016 | 6.046 | | 2015 | 5.958 | | 2014 | 5.71 | | 2013 | 5.537 | | 2012 | 5.368 | | 2011 | 5.122 | | 2010 | 4.947 | | 2009 | 4.784 | | 2008 | 4.648 | | 2007 | 4.566 | | 2006 | 4.437 | | 2005 | 4.303 | | 2004 | 4.273 | | 2003 | 4.293 | | 2002 | 4.411 | | 2001 | 4.436 | | 2000 | 4.663 | | 1999 | 4.706 | | 1998 | 4.774 | | 1997 | 4.788 | | 1996 | 4.791 | | 1995 | 4.636 | | 1994 | 4.629 | | 1993 | 4.579 | | 1992 | 4.578 | | 1991 | 4.57 | | 1990 | 4.486 | | 1989 | 4.495 | | 1988 | 4.491 | | 1987 | 4.504 | | 1986 | 4.493 | | 1985 | 4.481 | | 1984 | 4.467 | | 1983 | 4.48 | | 1982 | 4.48 | | 1981 | 4.502 | | 1980 | 4.502 | | 1979 | 4.471 | | 1978 | 4.657 | | 1977 | 4.838 | | 1976 | 5.075 | | 1975 | 5.123 | | 1974 | 5.233 | | 1973 | 5.245 | | 1972 | 5.333 | | 1971 | 5.338 | | 1970 | 5.427 | | 1969 | 5.507 | | 1968 | 5.594 | | 1967 | 5.801 | | 1966 | 6.118 | | 1965 | 5.971 | | 1964 | 5.901 | | 1963 | 6.044 | | 1962 | 6.278 | | 1961 | 6.317 | | 1960 | 6.443 |
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