| Year | Value | | 2016 | 5.4 | | 2015 | 6.9 | | 2014 | 7.6 | | 2013 | 11.5 | | 2012 | 12.2 | | 2011 | 13.3 | | 2010 | 8.2 | | 2009 | 9.3 | | 2007 | 8.3 | | 2006 | 7.4 | | 2005 | 7 | | 2004 | 7.7 | | 2002 | 11.39 | | 2001 | 13.213 | | 2000 | 11.48 | | 1999 | 13.95 | | 1998 | 7.74 | | 1997 | 9.67 | | 1996 | 8.283 | | 1995 | 7.9 | | 1994 | 7.6 | | 1993 | 5.527 | | 1992 | 3.837 | | 1991 | 3.509 | | 1990 | 2.831 | | 1989 | 2.81 | | 1988 | 4.27 | | 1986 | 6.28 | | 1983 | 9.25 | | 1982 | 9.6 | | 1981 | 8.7 | | 1980 | 4.9 | | 1976 | 8.79 | | 1975 | 8.21 | | 1974 | 9.6 |
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