| Year | Value | | 2022 | 18.940364574056 | | 2021 | 18.388116366935 | | 2020 | 17.909898489496 | | 2019 | 17.656290127877 | | 2018 | 18.014083738484 | | 2017 | 17.727390431331 | | 2016 | 16.430118320467 | | 2015 | 16.290968087047 | | 2014 | 16.828799153089 | | 2013 | 15.469636001974 | | 2012 | 15.595346340688 | | 2011 | 15.240327222996 | | 2010 | 15.16064072297 | | 2009 | 17.685313321309 | | 2008 | 20.445948952672 | | 2007 | 20.566535654127 | | 2006 | 19.57121151182 | | 2005 | 18.485339424201 | | 2004 | 16.357392607393 | | 2003 | 14.672870125638 | | 2002 | 12.881354369229 | | 2001 | 12.171200309308 | | 2000 | 14.725749611818 | | 1999 | 15.057353966878 | | 1998 | 18.080240838937 | | 1997 | 18.2831060982 | | 1996 | 12.0896762347 |
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