| Year | Value | | 2021 | 20.626135595316 | | 2020 | 20.278213339967 | | 2019 | 20.482815091351 | | 2018 | 19.912689171551 | | 2017 | 20.523224655166 | | 2016 | 20.333029961508 | | 2015 | 20.077803641444 | | 2014 | 19.566620772754 | | 2013 | 19.68628223533 | | 2012 | 18.385335264665 | | 2011 | 17.882715147573 | | 2010 | 17.871431557046 | | 2009 | 18.586498644158 | | 2008 | 21.973879820444 | | 2007 | 21.760613299221 | | 2006 | 21.812020406639 | | 2005 | 20.885521991508 | | 2004 | 21.029570986564 | | 2003 | 20.115186152398 | | 2002 | 18.147665811376 | | 2001 | 19.780494127026 | | 2000 | 20.457993065215 | | 1999 | 20.032295734015 | | 1998 | 19.37550127479 | | 1997 | 18.522634900134 | | 1996 | 12.06859737767 | | 1995 | 11.156664265327 | | 1994 | 21.23896499239 | | 1993 | 15.213783874205 | | 1992 | 16.903386454183 | | 1991 | 22.333824613117 | | 1990 | 23.833128445097 |
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