| Year | Value | | 2021 | 19.401846744636 | | 2020 | 16.301768762625 | | 2019 | 15.614991835231 | | 2018 | 15.680329994837 | | 2017 | 15.190631331102 | | 2016 | 16.176124450638 | | 2015 | 16.626168399977 | | 2014 | 17.318584203865 | | 2013 | 16.953650644203 | | 2012 | 15.524591466591 | | 2011 | 15.224210588949 | | 2010 | 18.238293965801 | | 2009 | 18.291676886701 | | 2008 | 17.720842005785 | | 2007 | 19.298165859025 | | 2006 | 16.860243738648 | | 2005 | 19.194501642229 | | 2004 | 19.723397824899 | | 1996 | 22.774340194469 | | 1995 | 24.381206221254 | | 1994 | 23.593002001915 | | 1993 | 23.915016578065 | | 1992 | 28.201158207995 | | 1991 | 31.991402086785 | | 1990 | 34.525843893562 |
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