| Year | Value | | 2022 | 6.011 | | 2021 | 6.577 | | 2020 | 6.796 | | 2019 | 6.464 | | 2018 | 6.994 | | 2017 | 8.869 | | 2016 | 11.068 | | 2015 | 12.447 | | 2014 | 13.897 | | 2013 | 16.188 | | 2012 | 15.531 | | 2011 | 12.682 | | 2010 | 10.771 | | 2009 | 9.432 | | 2008 | 7.553 | | 2007 | 7.966 | | 2006 | 7.649 | | 2005 | 7.582 | | 2004 | 6.319 | | 2003 | 6.132 | | 2002 | 4.497 | | 2001 | 6.8 | | 2000 | 3.806 | | 1999 | 4.579 | | 1998 | 4.64 | | 1997 | 6.571 | | 1996 | 7.298 | | 1995 | 7.064 | | 1994 | 6.709 | | 1993 | 5.301 | | 1992 | 3.985 | | 1991 | 6.01 | | 1990 | 4.645 | | 1989 | 5.13 | | 1988 | 6.008 | | 1987 | 7.377 | | 1986 | 8.698 | | 1985 | 8.18 | | 1983 | 7.3 | | 1979 | 7.94 | | 1978 | 7.89 | | 1976 | 6.4 | | 1975 | 4.5 | | 1974 | 1.77 |
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