| Year | Value | | 2022 | 22.115171308667 | | 2021 | 19.693529596734 | | 2020 | 20.022112045672 | | 2019 | 19.914427943866 | | 2018 | 20.286121377721 | | 2017 | 20.129701232229 | | 2016 | 19.867257761968 | | 2015 | 19.43746508762 | | 2014 | 20.353617753207 | | 2013 | 20.627869014934 | | 2012 | 21.965284105859 | | 2011 | 21.453460872023 | | 2010 | 21.088964089144 | | 2009 | 21.541484007866 | | 2008 | 24.068771088743 | | 2007 | 21.731474776024 | | 2006 | 19.403590924953 | | 2005 | 18.990049314323 | | 2004 | 17.116325321902 | | 2003 | 16.858250729215 | | 2002 | 17.111574031088 | | 1999 | 17.882547788984 | | 1998 | 17.231535306539 | | 1997 | 19.405487463984 | | 1995 | 18.351209036907 | | 1994 | 18.854935146609 | | 1993 | 20.247703340671 | | 1992 | 20.651429555534 | | 1991 | 18.415419378318 | | 1990 | 18.676721475858 |
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