| Year | Value | | 2020 | 3.6795989237173 | | 2019 | 3.692284970554 | | 2018 | 3.761416127301 | | 2017 | 3.8766639368491 | | 2016 | 5.8890808653542 | | 2015 | 4.5265032884435 | | 2014 | 4.5550346675071 | | 2013 | 4.5855641483244 | | 2012 | 4.6190183712093 | | 2011 | 4.657650127393 | | 2010 | 4.739166521503 | | 2009 | 4.8329876223268 | | 2008 | 4.7995392442325 | | 2007 | 4.9042045380239 | | 2006 | 5.0173097185289 | | 2005 | 6.2826985903909 | | 2004 | 5.8608058608059 | | 2003 | 6.4442691174965 | | 2002 | 6.6308066035397 | | 2001 | 6.191594750549 | | 2000 | 5.8224548263519 | | 1999 | 6.0031834023985 | | 1998 | 6.4384857819737 | | 1997 | 6.672171515357 | | 1996 | 6.9421664136463 | | 1995 | 7.0901871809416 | | 1994 | 4.0595289322627 | | 1993 | 3.3408781498217 | | 1992 | 3.4307965451879 | | 1991 | 3.52926644197 |
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