| Year | Value | | 2023 | 9.908 | | 2022 | 10.099 | | 2021 | 9.555 | | 2020 | 10.046 | | 2019 | 9.979 | | 2018 | 9.921 | | 2017 | 10.031 | | 2016 | 9.969 | | 2015 | 10.596 | | 2014 | 11.131 | | 2013 | 11.367 | | 2012 | 11.397 | | 2011 | 11.33 | | 2010 | 11.223 | | 2009 | 10.98 | | 2008 | 11.226 | | 2007 | 10.783 | | 2006 | 11.972 | | 2005 | 15.436 | | 2004 | 16.259 | | 2003 | 16.921 | | 2002 | 17.352 | | 2001 | 17.59 | | 2000 | 17.318 | | 1999 | 17.275 | | 1998 | 17.499 | | 1997 | 18.001 | | 1996 | 19.088 | | 1995 | 19.868 | | 1994 | 20.565 | | 1993 | 21.151 | | 1992 | 21.66 | | 1991 | 22.325 | | 1990 | 23.526 | | 1989 | 24.362 | | 1988 | 24.694 | | 1987 | 24.577 | | 1986 | 24.54 | | 1985 | 25.036 | | 1984 | 25.728 | | 1983 | 26.447 | | 1982 | 27.092 | | 1981 | 27.75 | | 1980 | 28.377 | | 1979 | 29.006 | | 1978 | 29.59 | | 1977 | 30.273 | | 1976 | 30.989 |
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