| Year | Value | | 2021 | 31.656757191065 | | 2020 | 31.640687771171 | | 2019 | 31.497581435711 | | 2018 | 31.14403715429 | | 2017 | 31.06368618124 | | 2016 | 31.03154579202 | | 2015 | 30.284281742652 | | 2014 | 30.083404310026 | | 2013 | 30.163755283077 | | 2012 | 29.585228277114 | | 2011 | 29.183473411863 | | 2010 | 29.006701271152 | | 2009 | 29.456666720234 | | 2008 | 29.328105163353 | | 2007 | 29.553087887894 | | 2006 | 29.810211001655 | | 2005 | 27.865717453838 | | 2004 | 26.387259549713 | | 2003 | 25.42304787311 | | 2002 | 25.631960403041 | | 2001 | 25.4069776785 | | 2000 | 25.51946904077 | | 1999 | 25.985504684462 | | 1998 | 28.106770372989 | | 1997 | 28.476384849021 | | 1996 | 30.115544699246 | | 1995 | 29.440596525624 | | 1994 | 40.818294309544 | | 1993 | 40.400469249683 | | 1992 | 40.657592363443 |
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