| Year | Value | | 2021 | 21.922693913567 | | 2020 | 22.035801280951 | | 2019 | 21.339226644212 | | 2018 | 22.826737181746 | | 2017 | 23.479512795023 | | 2016 | 23.503897813433 | | 2015 | 22.388436122918 | | 2014 | 22.048271853247 | | 2013 | 21.748807167295 | | 2012 | 21.075770173507 | | 2011 | 20.599108744614 | | 2010 | 19.657812330458 | | 2009 | 18.671035858419 | | 2008 | 20.682500031513 | | 2007 | 21.609388744778 | | 2006 | 22.123019917615 | | 2005 | 21.439380965541 | | 2004 | 20.330597877711 | | 2003 | 20.204592319533 | | 2002 | 19.962688228886 | | 2001 | 20.660662272767 | | 2000 | 20.715063281985 | | 1999 | 22.668383189092 | | 1998 | 23.663940220399 | | 1997 | 22.250659004196 | | 1996 | 18.656339034961 | | 1995 | 19.3430706966 |
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