| Year | Value | | 2018 | 4.5941897509099 | | 2017 | 5.4097535307528 | | 2016 | 5.4494391470471 | | 2015 | 5.4881652817758 | | 2014 | 5.5970299910578 | | 2013 | 5.564824915673 | | 2012 | 5.661108643126 | | 2011 | 5.6611314047004 | | 2010 | 5.707752350539 | | 2009 | 5.801668006087 | | 2008 | 5.889601901955 | | 2007 | 5.9460301745008 | | 2006 | 6.0269942207 | | 2005 | 6.1155863923658 | | 2004 | 6.1937837813928 | | 2003 | 6.3004792352401 | | 2002 | 6.3724445908037 | | 2001 | 6.5762408520062 | | 2000 | 6.5322972832646 | | 1999 | 6.6692990226787 | | 1998 | 6.8603039967088 | | 1997 | 7.0621173031409 | | 1996 | 7.2903074067915 | | 1995 | 7.3098784857335 | | 1994 | 7.5428237789877 | | 1993 | 7.6093586081947 | | 1992 | 7.7470955582045 | | 1991 | 8.0079697420957 | | 1990 | 8.1962908553538 | | 1985 | 8.8777850206841 | | 1980 | 9.5198467242852 |
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