| Year | Value | | 2020 | 2.7132452819674 | | 2019 | 2.5963653151698 | | 2018 | 2.5773536179743 | | 2017 | 2.4246552923015 | | 2016 | 2.4901488638727 | | 2015 | 2.5178520493222 | | 2014 | 2.5043224527482 | | 2013 | 2.5176795146479 | | 2012 | 3.1560653424337 | | 2011 | 3.2414497488002 | | 2010 | 3.4848480006766 | | 2009 | 3.4766158591163 | | 2008 | 3.4851589070279 | | 2007 | 3.2926580579495 | | 2006 | 3.5313893230369 | | 2005 | 3.285432576207 | | 2004 | 3.1932886083286 | | 2003 | 3.7115399103646 | | 2002 | 3.8095841384209 | | 2001 | 3.8742838049048 | | 2000 | 4.1550940470472 | | 1999 | 4.2740634384656 | | 1998 | 4.4506790809385 | | 1997 | 4.6277138717165 | | 1996 | 4.6839799363732 | | 1995 | 4.4892650567876 | | 1994 | 3.4808642576958 | | 1993 | 3.6605973498154 | | 1992 | 3.7684436237404 | | 1991 | 3.9999264270561 |
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