| Year | Value | | 2024 | 10.375 | | 2023 | 10.147 | | 2022 | 10.286 | | 2021 | 15.253 | | 2020 | 10.403 | | 2019 | 10.047 | | 2018 | 9.866 | | 2017 | 9.91 | | 2016 | 9.44 | | 2015 | 9.123 | | 2014 | 8.884 | | 2013 | 8.516 | | 2012 | 8.245 | | 2011 | 8.165 | | 2010 | 8.151 | | 2009 | 8.212 | | 2008 | 8.002 | | 2007 | 7.84 | | 2006 | 7.701 | | 2005 | 7.364 | | 2004 | 7.324 | | 2003 | 7.103 | | 2002 | 7.326 | | 2001 | 7.28 | | 2000 | 7.354 | | 1999 | 7.237 | | 1998 | 7.214 | | 1997 | 7.262 | | 1996 | 7.334 | | 1995 | 7.373 | | 1994 | 7.353 | | 1993 | 7.38 | | 1992 | 7.255 | | 1991 | 7.17 | | 1990 | 6.982 | | 1989 | 6.942 | | 1988 | 6.789 | | 1987 | 6.733 | | 1986 | 6.647 | | 1985 | 6.581 | | 1984 | 6.434 | | 1983 | 6.303 | | 1982 | 6.256 | | 1981 | 6.377 | | 1980 | 6.314 | | 1979 | 6.251 | | 1978 | 6.188 | | 1977 | 6.215 | | 1976 | 6.27 |
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