| Year | Value | | 2020 | 1.7630350811471 | | 2019 | 1.6273228014982 | | 2018 | 1.7350989939387 | | 2017 | 1.8094945471385 | | 2016 | 1.8005951486024 | | 2015 | 1.8710724682362 | | 2014 | 1.937896093633 | | 2013 | 1.9903672556017 | | 2012 | 2.0449345206218 | | 2011 | 2.1439383372338 | | 2010 | 2.2560813624484 | | 2009 | 2.3977074450261 | | 2008 | 2.399618561473 | | 2007 | 2.5189383415755 | | 2006 | 2.6696474763878 | | 2005 | 2.5237229961639 | | 2004 | 2.6008842277154 | | 2003 | 2.7050772538609 | | 2002 | 2.8195475163917 | | 2001 | 2.9618425497046 | | 2000 | 3.0652094350254 | | 1999 | 2.6740803792507 | | 1998 | 2.7622406054335 | | 1997 | 2.8443728557344 | | 1996 | 2.9331816295081 | | 1995 | 2.9948394276769 | | 1994 | 2.8396527727997 | | 1993 | 3.0515092995633 | | 1992 | 3.1461058891444 | | 1991 | 3.6650933629764 |
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