| Year | Value | | 2020 | 15.981102044679 | | 2019 | 14.814064626389 | | 2018 | 14.728848045185 | | 2017 | 14.702124242513 | | 2016 | 15.020630435532 | | 2015 | 14.951025618055 | | 2014 | 15.431644034003 | | 2013 | 14.930514996279 | | 2012 | 14.9212805171 | | 2011 | 14.608237222507 | | 2010 | 15.499090812401 | | 2009 | 16.530351582939 | | 2008 | 17.152210648873 | | 2007 | 24.689851870781 | | 2006 | 24.554864454728 | | 2005 | 24.415165367034 | | 2004 | 20.995502342137 | | 2003 | 18.217936835116 | | 2002 | 17.481601413012 | | 2001 | 18.680484623725 | | 2000 | 18.998116258523 | | 1999 | 18.108786321236 | | 1998 | 18.123462512032 | | 1997 | 19.055729663442 | | 1996 | 21.689673426175 | | 1995 | 20.708634695739 | | 1994 | 19.990820003213 | | 1993 | 20.916229018639 | | 1992 | 22.972246842455 | | 1991 | 17.949966193374 | | 1990 | 18.006591575821 |
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