| Year | Value | | 2014 | 2.89 | | 2013 | 2.59 | | 2012 | 2.53 | | 2011 | 2.81 | | 2010 | 2.99 | | 2009 | 3.09 | | 2008 | 3.21 | | 2007 | 3.32 | | 2006 | 3.74 | | 2005 | 3.92 | | 2004 | 4.12 | | 2003 | 4.19 | | 2002 | 4.2 | | 2001 | 4.45 | | 2000 | 4.81 | | 1999 | 5.6900000572 | | 1998 | 5.7100000381 | | 1997 | 5.8600001335 | | 1996 | 5.6700000763 | | 1995 | 7.6599998474 | | 1994 | 9.6758003235 | | 1993 | 9.3910999298 | | 1992 | 10.0087003708 | | 1991 | 9.7981004715 | | 1990 | 9.7974996567 | | 1989 | 9.9956998825073 | | 1988 | 9.9934997558594 | | 1987 | 10.107399940491 | | 1986 | 10.015000343323 | | 1985 | 9.9734001159668 | | 1980 | 10.170800209045 |
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