| Year | Value | | 2013 | 15.309013006354 | | 2012 | 13.311432116805 | | 2011 | 10.646377707311 | | 2010 | 4.5911664374481 | | 2009 | 3.8221573425058 | | 2008 | 4.173763519337 | | 2007 | 6.6993334687398 | | 2006 | 8.4289944815935 | | 2005 | 8.6827585862649 | | 2004 | 7.2692278267979 | | 2003 | 9.1091436167677 | | 2002 | 10.674895873826 | | 2001 | 11.213288056668 | | 2000 | 12.505896208152 | | 1999 | 17.644159242354 | | 1998 | 16.960862592857 | | 1997 | 16.947654377128 | | 1996 | 15.759987444583 | | 1995 | 14.8413410277 | | 1994 | 12.454800667738 | | 1993 | 11.43864255698 | | 1992 | 11.305759584994 | | 1991 | 8.8967835757682 | | 1990 | 8.04122502693 | | 1989 | 9.6243096931102 | | 1988 | 10.175544295662 | | 1987 | 10.659711647634 | | 1986 | 12.967658081623 | | 1985 | 13.090096243201 | | 1984 | 11.983151948659 | | 1983 | 12.534533986177 | | 1982 | 12.075824778037 | | 1981 | 13.662816171442 | | 1980 | 10.774883162294 | | 1979 | 6.5221333037053 | | 1978 | 7.1704662928713 | | 1977 | 7.3704788770107 | | 1976 | 8.1569761614821 | | 1975 | 9.4395298184307 | | 1974 | 8.3334286694886 | | 1973 | 9.9646971443102 | | 1972 | 9.3386419756206 | | 1971 | 6.1843932358469 |
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