| Year | Value | | 2014 | 3.98 | | 2013 | 3.99 | | 2012 | 4.19 | | 2011 | 4.32 | | 2010 | 4.44 | | 2009 | 4.58 | | 2008 | 4.67 | | 2007 | 4.83 | | 2006 | 5.12 | | 2005 | 5.19 | | 2004 | 5.26 | | 2003 | 5.48 | | 2002 | 5.54 | | 2001 | 5.34 | | 2000 | 5.33 | | 1999 | 5.3899998665 | | 1998 | 5.5500001907 | | 1997 | 6.4800000191 | | 1996 | 7.9299998283 | | 1995 | 8.3000001907 | | 1994 | 8.6632995605 | | 1993 | 9.394200325 | | 1992 | 10.7540998459 | | 1991 | 10.9472999573 | | 1990 | 12.48279953 | | 1989 | 12.285099983215 | | 1988 | 12.216199874878 | | 1987 | 11.945699691773 | | 1986 | 11.955699920654 | | 1985 | 11.834199905396 | | 1980 | 9.1763000488281 |
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