| Year | Value | | 2021 | 18.518518518519 | | 2020 | 18.518518518519 | | 2019 | 18.518518518519 | | 2018 | 18.518518518519 | | 2017 | 18.518518518519 | | 2016 | 18.518518518519 | | 2015 | 18.518518518519 | | 2014 | 18.518518518519 | | 2013 | 18.518518518519 | | 2012 | 18.518518518519 | | 2011 | 18.518518518519 | | 2010 | 18.518518518519 | | 2009 | 18.518518518519 | | 2008 | 18.518518518519 | | 2007 | 18.518518518519 | | 2006 | 18.518518518519 | | 2005 | 18.518518518519 | | 2004 | 18.518518518519 | | 2003 | 18.518518518519 | | 2002 | 18.518518518519 | | 2001 | 18.518518518519 | | 2000 | 18.518518518519 | | 1999 | 18.518518518519 | | 1998 | 18.518518518519 | | 1997 | 18.518518518519 | | 1996 | 18.518518518519 | | 1995 | 18.518518518519 | | 1994 | 18.518518518519 | | 1993 | 18.518518518519 | | 1992 | 18.518518518519 | | 1991 | 18.518518518519 | | 1990 | 18.518518518519 |
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