| Year | Value | | 2023 | 10.1 | | 2022 | 11.3 | | 2021 | 14.6 | | 2020 | 11.6 | | 2019 | 10.5 | | 2018 | 10.4 | | 2017 | 10.4 | | 2016 | 10.3 | | 2015 | 10.1 | | 2014 | 9.6 | | 2013 | 9.5 | | 2012 | 9.5 | | 2011 | 9.4 | | 2010 | 9.1 | | 2009 | 9.5 | | 2008 | 9.3 | | 2007 | 9.7 | | 2006 | 9.7 | | 2005 | 9.5 | | 2004 | 9.3 | | 2003 | 9.3 | | 2002 | 9 | | 2001 | 8.9 | | 2000 | 9 | | 1999 | 8.9 | | 1998 | 8.7 | | 1997 | 8.5 | | 1996 | 8.2 | | 1995 | 8 | | 1994 | 7.843 | | 1993 | 7.638 | | 1992 | 7.547 | | 1991 | 7.356 | | 1990 | 7.263 | | 1989 | 7.209 | | 1988 | 7.33 | | 1987 | 7.359 | | 1986 | 7.46 | | 1985 | 7.551 | | 1984 | 7.527 | | 1983 | 7.428 | | 1982 | 7.332 | | 1981 | 7.322 | | 1980 | 7.173 | | 1979 | 7.386 | | 1978 | 7.168 | | 1977 | 7.143 | | 1976 | 6.964 | | 1975 | 6.943 |
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