| Year | Value | | 2020 | 2.0982545647082 | | 2019 | 1.9256060537777 | | 2018 | 1.870007961579 | | 2017 | 1.8897366792662 | | 2016 | 1.9557354647244 | | 2015 | 2.0797490849104 | | 2014 | 2.1962503931055 | | 2013 | 2.3463572803223 | | 2012 | 2.6420658481509 | | 2011 | 3.0961669453217 | | 2010 | 3.6958489325681 | | 2009 | 4.1677640300682 | | 2008 | 4.3548760713458 | | 2007 | 4.5510839810559 | | 2006 | 4.7536352667143 | | 2005 | 4.8539488984702 | | 2004 | 4.9459680468482 | | 2003 | 5.2412152797911 | | 2002 | 5.3902241678476 | | 2001 | 5.7326590063311 | | 2000 | 5.8574786183688 | | 1999 | 5.954556416625 | | 1998 | 6.0755807316572 | | 1997 | 6.1843557747552 | | 1996 | 6.2756874104994 | | 1995 | 6.3760823699326 | | 1994 | 4.7258341097204 | | 1993 | 4.927280380555 | | 1992 | 5.2984944706426 | | 1991 | 4.7426925737611 |
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