| Year | Value | | 2020 | 4.3576708688351 | | 2019 | 4.4022000079854 | | 2018 | 4.4839544935927 | | 2017 | 4.5594268270312 | | 2016 | 4.6444311974826 | | 2015 | 4.7291057594354 | | 2014 | 4.8170922822047 | | 2013 | 4.9083514592555 | | 2012 | 5.001020735061 | | 2011 | 5.0973289759265 | | 2010 | 5.1992111375077 | | 2009 | 5.306601107604 | | 2008 | 5.4225945643744 | | 2007 | 5.5523656951605 | | 2006 | 5.6928458139048 | | 2005 | 5.8350129727265 | | 2004 | 5.9719346846263 | | 2003 | 6.104607725757 | | 2002 | 6.2354468004333 | | 2001 | 6.3681542957912 | | 2000 | 6.502877435116 | | 1999 | 6.6390988927464 | | 1998 | 6.7704309948338 | | 1997 | 6.916611932496 | | 1996 | 7.497408742307 | | 1995 | 7.6572288991644 | | 1994 | 2.5698132602364 | | 1993 | 2.1585608816263 | | 1992 | 2.2108994954966 | | 1991 | 3.2474594756885 |
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