| Year | Value | | 2020 | 8.9876891754093 | | 2019 | 8.9337183064016 | | 2018 | 8.9928241927902 | | 2017 | 8.7956615872147 | | 2016 | 8.8665543928756 | | 2015 | 8.27667966803 | | 2014 | 8.3064449617158 | | 2013 | 8.6055542414748 | | 2012 | 8.8214751745664 | | 2011 | 8.4718302709675 | | 2010 | 8.5470572415914 | | 2009 | 8.4066390454659 | | 2008 | 9.9062186485758 | | 2007 | 10.065829183465 | | 2006 | 9.5620290964003 | | 2005 | 10.138352252314 | | 2004 | 9.8263425069664 | | 2003 | 10.014749650428 | | 2002 | 10.79217357103 | | 2001 | 10.709652632477 | | 2000 | 10.598667520898 | | 1999 | 10.727195104419 | | 1998 | 11.681049955289 | | 1997 | 12.483357947795 | | 1996 | 12.564186399329 | | 1995 | 13.527746582373 | | 1994 | 12.889165216225 | | 1993 | 15.454381385222 | | 1992 | 15.178677116563 | | 1991 | 17.084467031155 | | 1990 | 17.024299361248 | | 1989 | 19.722305576592 | | 1988 | 20.018178714146 |
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