| Year | Value | | 2020 | 2.7985068693803 | | 2019 | 2.8139692975754 | | 2018 | 2.7943274650784 | | 2017 | 2.7474111072836 | | 2016 | 2.7014653776414 | | 2015 | 2.7356939496175 | | 2014 | 2.7254258253588 | | 2013 | 2.7682331807221 | | 2012 | 2.8279739869718 | | 2011 | 2.8749380279666 | | 2010 | 2.9071072247617 | | 2009 | 3.1724488196584 | | 2008 | 3.2697725050116 | | 2007 | 3.3203396607786 | | 2006 | 3.4703827279132 | | 2005 | 3.3495583592631 | | 2004 | 3.5066526429856 | | 2003 | 3.6002149605271 | | 2002 | 3.7543025132457 | | 2001 | 3.6937228498746 | | 2000 | 3.266312390117 | | 1999 | 3.2793173502942 | | 1998 | 3.4900809077741 | | 1997 | 3.5469166575255 | | 1996 | 3.5619586001821 | | 1995 | 3.2845815281596 | | 1994 | 2.3369504253467 | | 1993 | 2.4206542092037 | | 1992 | 2.5260753231574 | | 1991 | 2.6837694097903 |
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