| Year | Value | | 2015 | 2.103 | | 2014 | 2.08 | | 2013 | 1.993 | | 2012 | 1.912 | | 2011 | 1.838 | | 2010 | 1.749 | | 2009 | 1.674 | | 2008 | 1.591 | | 2007 | 1.543 | | 2006 | 1.462 | | 2005 | 1.41 | | 2004 | 1.371 | | 2003 | 1.329 | | 2002 | 1.296 | | 2001 | 1.298 | | 2000 | 1.306 | | 1991 | 1.561 | | 1990 | 1.565 | | 1987 | 1.485 | | 1986 | 1.419 | | 1985 | 1.362 | | 1984 | 1.28 | | 1983 | 1.207 | | 1982 | 1.17 | | 1981 | 1.089 | | 1980 | 1.033 |
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