| Year | Value | | 2020 | 3.9689572796493 | | 2019 | 4.1550120369982 | | 2018 | 3.7485204423623 | | 2017 | 4.298228907368 | | 2016 | 4.558833222882 | | 2015 | 4.9751715249611 | | 2014 | 5.6660364529729 | | 2013 | 6.2693304355095 | | 2012 | 6.5629996829304 | | 2011 | 6.2470955245921 | | 2010 | 6.3707303855013 | | 2009 | 6.4590760540131 | | 2008 | 6.7776656986613 | | 2007 | 6.9359654176513 | | 2006 | 7.8484943618114 | | 2005 | 8.3165815403358 | | 2004 | 8.6158887955066 | | 2003 | 9.0369180592789 | | 2002 | 9.0287017677248 | | 2001 | 9.1774215442885 | | 2000 | 12.52671074992 | | 1999 | 11.531067621967 | | 1998 | 11.809220089329 | | 1997 | 12.10474464452 | | 1996 | 11.959510505691 | | 1995 | 12.432652437757 | | 1994 | 10.182967561139 | | 1993 | 10.88056404844 | | 1992 | 11.735918364952 | | 1991 | 12.556267774967 |
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