| Year | Value | | 2024 | 7.498 | | 2023 | 7.614 | | 2022 | 7.779 | | 2021 | 8.782 | | 2020 | 8.138 | | 2019 | 8.281 | | 2018 | 8.321 | | 2017 | 8.587 | | 2016 | 8.816 | | 2015 | 9.18 | | 2014 | 9.621 | | 2013 | 10.233 | | 2012 | 10.946 | | 2011 | 11.694 | | 2010 | 12.745 | | 2009 | 13.983 | | 2008 | 14.714 | | 2007 | 15.204 | | 2006 | 15.501 | | 2005 | 15.893 | | 2004 | 16.246 | | 2003 | 16.743 | | 2002 | 16.339 | | 2001 | 17.426 | | 2000 | 16.254 | | 1999 | 15.893 | | 1998 | 15.011 | | 1997 | 14.349 | | 1996 | 14.009 | | 1995 | 12.831 | | 1994 | 12.128 | | 1993 | 11.381 | | 1992 | 10.416 | | 1991 | 9.834 | | 1990 | 9.325 | | 1989 | 8.932 | | 1988 | 8.696 | | 1987 | 8.613 | | 1986 | 8.663 | | 1985 | 8.865 | | 1984 | 9.156 | | 1983 | 9.605 | | 1982 | 9.818 | | 1981 | 10.125 | | 1980 | 10.588 | | 1979 | 12.66 | | 1978 | 12.69 | | 1977 | 11.602 | | 1976 | 11.306 |
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