| Year | Value | | 2022 | 8.299 | | 2021 | 9.325 | | 2020 | 11.006 | | 2019 | 7.533 | | 2018 | 7.647 | | 2017 | 7.29 | | 2016 | 6.926 | | 2015 | 6.605 | | 2014 | 6.761 | | 2013 | 6.361 | | 2012 | 6.79 | | 2011 | 7.492 | | 2010 | 8.547 | | 2009 | 11.313 | | 2008 | 9.285 | | 2007 | 8.427 | | 2006 | 9.024 | | 2005 | 9.34 | | 2004 | 10.162 | | 2003 | 9.765 | | 2002 | 10.168 | | 2001 | 10.39 | | 2000 | 10.491 | | 1999 | 11.158 | | 1998 | 7.307 | | 1997 | 7.136 | | 1996 | 7.414 | | 1995 | 4.7 | | 1994 | 5.87 | | 1993 | 4.49 | | 1992 | 4.35 | | 1991 | 5.23 | | 1990 | 5.63 | | 1989 | 5.29 | | 1988 | 6.23 | | 1986 | 8.71 | | 1984 | 13.91 | | 1983 | 14.65 | | 1982 | 19.6 | | 1981 | 11.35 | | 1980 | 10.46 | | 1979 | 13.63 | | 1978 | 14.26 | | 1977 | 11.83 | | 1976 | 12.75 | | 1975 | 14.69 |
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