| Year | Value | | 2015 | 6.370840183514 | | 2014 | 6.3745121056024 | | 2013 | 6.5975869200309 | | 2012 | 6.7246450417144 | | 2011 | 6.7819419785553 | | 2010 | 6.8805812297307 | | 2009 | 7.0491559799664 | | 2008 | 7.0831412324308 | | 2007 | 7.3443111614322 | | 2006 | 7.4699898182194 | | 2005 | 7.5330418277425 | | 2004 | 7.6334918845352 | | 2003 | 7.6724579967921 | | 2002 | 7.6953709037108 | | 2001 | 7.97219927486 | | 2000 | 7.7819783973853 | | 1999 | 7.76269017952 | | 1998 | 7.9817936539681 | | 1997 | 8.2835790661707 | | 1996 | 8.7733972778105 | | 1995 | 9.142772643824 | | 1994 | 9.3544796681032 | | 1993 | 9.6295587138927 | | 1992 | 9.7927533883995 | | 1991 | 10.026136187064 | | 1990 | 10.25568593434 | | 1989 | 10.732352945241 | | 1988 | 10.716147920421 | | 1987 | 10.683360685578 | | 1986 | 10.650990481632 | | 1985 | 10.144377768615 |
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