| Year | Value | | 2018 | 4.2775089359279 | | 2017 | 5.2077737321785 | | 2016 | 5.2567665650868 | | 2015 | 5.30688427996 | | 2014 | 5.4390059195395 | | 2013 | 5.4040301963521 | | 2012 | 5.4828316205542 | | 2011 | 5.5328028648672 | | 2010 | 5.5788600480361 | | 2009 | 5.6774062212234 | | 2008 | 5.7949618759416 | | 2007 | 5.8850165373848 | | 2006 | 5.9449004330218 | | 2005 | 6.0442133137096 | | 2004 | 6.1312874224573 | | 2003 | 6.2578626420927 | | 2002 | 6.4052616903085 | | 2001 | 6.5315764333074 | | 2000 | 6.6477603970208 | | 1999 | 6.8662925281692 | | 1998 | 7.0593005919231 | | 1997 | 7.2176856759971 | | 1996 | 7.4831517848061 | | 1995 | 7.4903745013666 | | 1994 | 7.6233800670329 | | 1993 | 7.7250604080029 | | 1992 | 7.8832518033054 | | 1991 | 8.0082188817761 | | 1990 | 8.2272443797373 | | 1985 | 9.0489583239263 | | 1980 | 9.8967860134078 | | 1970 | 9.6515974714603 |
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