| Year | Value | | 2018 | 4.7160662195433 | | 2017 | 5.3635821789645 | | 2016 | 5.4099706831409 | | 2015 | 5.4662484484445 | | 2014 | 5.6773667900616 | | 2013 | 5.7810015126655 | | 2012 | 5.9010343804754 | | 2011 | 5.943396293495 | | 2010 | 6.02063550878 | | 2009 | 6.164815159565 | | 2008 | 6.2322499818098 | | 2007 | 6.410943846871 | | 2006 | 6.5225705938775 | | 2005 | 6.6116915869876 | | 2004 | 6.70349338244 | | 2003 | 6.7818375717642 | | 2002 | 6.8577776482022 | | 2001 | 7.000880820783 | | 2000 | 7.007940701717 | | 1999 | 7.0881732051157 | | 1998 | 7.292636718467 | | 1997 | 7.5381281962499 | | 1996 | 7.898554823422 | | 1995 | 8.124053960083 | | 1994 | 8.3198182860904 | | 1993 | 8.5164209647378 | | 1992 | 8.6928702860979 | | 1991 | 9.02234526468 | | 1990 | 9.1597174075931 | | 1989 | 9.5626994396175 | | 1988 | 9.6217137326394 | | 1987 | 9.6531236940313 | | 1986 | 9.6708833574371 | | 1985 | 9.5862933037203 | | 1980 | 9.1484962327555 |
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