| Year | Value | | 2016 | 2.67 | | 2014 | 1.98 | | 2013 | 2.598 | | 2011 | 3.31 | | 2008 | 1.4 | | 2007 | 1.4 | | 2006 | 2.42 | | 2005 | 1.4 | | 2004 | 1 | | 2003 | 0.8 | | 2002 | 0.5 | | 2001 | 1.6 | | 2000 | 0.6 | | 1999 | 0.8 | | 1996 | 3.54 | | 1995 | 4.4 | | 1994 | 4.7 | | 1993 | 4.9 | | 1992 | 4.3 | | 1991 | 4.1 | | 1990 | 2.1 | | 1989 | 2 | | 1988 | 3.3 | | 1987 | 5.8 | | 1986 | 7.7 | | 1985 | 8 | | 1984 | 7.4 | | 1983 | 7.1 | | 1982 | 5.9 | | 1981 | 4.6 | | 1980 | 2.9 | | 1979 | 2.3 | | 1978 | 3.1 | | 1977 | 3.7 | | 1976 | 3.5 | | 1975 | 3.1 |
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