| Year | Value | | 2021 | 2.791120844538 | | 2020 | 2.8088952608017 | | 2019 | 2.8266207876558 | | 2018 | 2.8444204691597 | | 2017 | 2.86394290918 | | 2016 | 2.883323261221 | | 2015 | 2.9009380385279 | | 2014 | 2.9194517469536 | | 2013 | 2.9379369968137 | | 2012 | 2.9564054371818 | | 2011 | 3.3897764010244 | | 2010 | 3.4075981442333 | | 2009 | 3.4220115599937 | | 2008 | 3.4357932844383 | | 2007 | 3.4502085798957 | | 2006 | 3.4646264419349 | | 2005 | 3.4790545703698 | | 2004 | 3.4934699929272 | | 2003 | 3.5078802822753 | | 2002 | 3.5222465587931 | | 2001 | 3.5366592241477 | | 2000 | 3.5510718895023 | | 1999 | 3.5691408713749 | | 1998 | 3.5872098532476 | | 1997 | 3.6052788351203 | | 1996 | 3.6233478169929 | | 1995 | 3.6414167988656 | | 1994 | 3.6594857733915 | | 1993 | 3.6775547552642 | | 1992 | 3.6956237371368 | | 1991 | 3.7136981757439 | | 1990 | 3.7317671841663 |
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