| Year | Value | | 2022 | 7.487 | | 2021 | 7.922 | | 2020 | 7.264 | | 2019 | 7.14 | | 2018 | 7.027 | | 2017 | 6.897 | | 2016 | 6.814 | | 2015 | 6.724 | | 2014 | 6.627 | | 2013 | 6.547 | | 2012 | 6.464 | | 2011 | 6.393 | | 2010 | 6.307 | | 2009 | 6.236 | | 2008 | 6.189 | | 2007 | 6.122 | | 2006 | 6.033 | | 2005 | 6.323 | | 2004 | 6.419 | | 2003 | 6.23 | | 2002 | 6.138 | | 2001 | 6.088 | | 2000 | 6.043 | | 1999 | 5.822 | | 1998 | 5.588 | | 1997 | 5.614 | | 1996 | 5.818 | | 1995 | 5.964 | | 1994 | 5.747 | | 1993 | 5.695 | | 1992 | 5.654 | | 1991 | 5.612 | | 1990 | 5.602 | | 1989 | 5.615 | | 1988 | 5.663 | | 1987 | 5.665 | | 1986 | 5.935 | | 1985 | 6.153 | | 1984 | 6.395 | | 1983 | 6.69 | | 1982 | 6.89 | | 1981 | 7.08 | | 1980 | 7.27 | | 1979 | 7.535 | | 1978 | 7.702 | | 1977 | 7.863 | | 1976 | 8.045 | | 1975 | 8.378 | | 1974 | 8.73 |
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