| Year | Value | | 2022 | 3.0043137723859 | | 2021 | 3.2504810493812 | | 2020 | 3.4808570941673 | | 2019 | 3.2974263140191 | | 2018 | 3.3326922839615 | | 2017 | 3.522388500103 | | 2016 | 3.9710449247921 | | 2015 | 4.5280001677868 | | 2014 | 4.3258672400794 | | 2013 | 3.206416324294 | | 2012 | 3.1665425912114 | | 2011 | 3.5745889549471 | | 2010 | 3.5147439754631 | | 2009 | 3.3743636837362 | | 2008 | 3.1372615742466 | | 2007 | 2.6104245571299 | | 2006 | 2.5012493984378 | | 2005 | 2.6452053014553 | | 2004 | 2.5275896811078 | | 2003 | 2.6249128826462 | | 2002 | 2.6178453791854 | | 2001 | 3.0087408798671 | | 2000 | 2.7067327986492 | | 1999 | 3.1240330690389 | | 1998 | 2.3218372451967 | | 1997 | 2.3013551429765 | | 1996 | 1.9036040237159 | | 1995 | 1.9478986305682 | | 1994 | 1.7512338730626 | | 1993 | 2.4645237583315 | | 1992 | 4.250418760469 | | 1991 | 5.6152549515381 | | 1990 | 6.4210026885972 |
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