| Year | Value | | 2013 | 8.2929429095356 | | 2012 | 6.7774665799939 | | 2011 | 6.3894136491497 | | 2010 | 4.7737864555815 | | 2009 | 5.0088617422115 | | 2008 | 5.8898862829686 | | 2007 | 4.9623106772864 | | 2006 | 6.9757071800738 | | 2005 | 7.4569748163983 | | 2004 | 7.3484790448865 | | 2003 | 6.1262776565219 | | 2002 | 5.3501642367349 | | 2001 | 4.3401482203815 | | 2000 | 3.8370084997874 | | 1999 | 4.400575219722 | | 1998 | 7.3172979994035 | | 1997 | 8.8938386959901 | | 1996 | 7.4092950321518 | | 1995 | 8.1749541351779 | | 1994 | 8.2377398462045 | | 1993 | 7.7131510540924 | | 1992 | 11.040482013682 | | 1991 | 8.630752374974 | | 1990 | 9.3737670466411 | | 1989 | 14.067824253403 | | 1988 | 12.196578316249 | | 1987 | 10.871451242088 | | 1986 | 7.3729218747866 | | 1985 | 8.0990718237609 | | 1984 | 9.1829114371397 | | 1983 | 10.462097462966 | | 1982 | 8.5353857523241 | | 1981 | 10.594399366628 | | 1980 | 12.12927384139 | | 1979 | 11.721781286749 | | 1978 | 13.392245441694 | | 1977 | 13.100977177877 | | 1976 | 18.716648900381 | | 1975 | 13.37643346033 | | 1974 | 20.500249888643 | | 1973 | 23.196199790113 | | 1972 | 20.557657467085 | | 1971 | 25.161593756509 |
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