| Year | Value | | 2020 | 3.3853476391882 | | 2019 | 3.4660230709813 | | 2018 | 3.4921648130046 | | 2017 | 3.4790066796928 | | 2016 | 3.4605514751746 | | 2015 | 3.4418340559316 | | 2014 | 3.4245643793692 | | 2013 | 3.4085931257493 | | 2012 | 3.395456588638 | | 2011 | 3.8230878882092 | | 2010 | 3.8125841091978 | | 2009 | 3.8189132762386 | | 2008 | 2.9227537071999 | | 2007 | 2.9251720105613 | | 2006 | 3.2856908164942 | | 2005 | 3.4340346473071 | | 2004 | 3.4379089094462 | | 2003 | 3.1983256484675 | | 2002 | 3.1920076608184 | | 2001 | 2.9997914098531 | | 2000 | 2.9400848245749 | | 1999 | 3.7679810271073 | | 1998 | 3.755316292205 | | 1997 | 4.1922468175005 | | 1996 | 3.9902893848053 | | 1995 | 4.1423979866588 | | 1994 | 3.003329691518 | | 1993 | 1.3595031210307 | | 1992 | 1.2407148006109 |
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