| Year | Value | | 2022 | 15.701004725782 | | 2021 | 14.585328260875 | | 2020 | 16.687409274031 | | 2019 | 24.217722912944 | | 2018 | 19.611448370925 | | 2017 | 22.389269331208 | | 2016 | 22.573422968522 | | 2015 | 19.645333518448 | | 2014 | 17.463773590509 | | 2013 | 17.239523513509 | | 2012 | 8.5209144001047 | | 2011 | 6.8975810915877 | | 2010 | 3.9221688147334 | | 2009 | 3.0477331087364 | | 2008 | 1.5583399962592 | | 2007 | 14.158666254104 | | 2006 | 10.905429441795 | | 2005 | 12.176025561071 | | 2004 | 15.63886274233 | | 2003 | 17.31279329294 | | 2002 | 21.620984047659 | | 2001 | 18.451264023604 | | 2000 | 14.153316175867 | | 1999 | 16.876034377247 | | 1998 | 19.328346337001 | | 1997 | 20.61856568022 | | 1996 | 20.208281641116 | | 1995 | 19.94392522423 | | 1994 | 20.78590192766 | | 1993 | 20.491641171569 | | 1992 | 18.644473169789 | | 1991 | 18.945076462718 | | 1990 | 19.751158160659 | | 1989 | 20.035393935077 | | 1988 | 20.149035960913 | | 1987 | 20.32499863625 | | 1986 | 21.900040618878 | | 1985 | 19.855292543303 | | 1984 | 21.406827254878 | | 1983 | 22.005953725467 | | 1982 | 22.86380564536 | | 1981 | 25.409612917183 | | 1980 | 27.83555015544 | | 1979 | 27.207946716559 | | 1977 | 30.513818566313 | | 1976 | 30.033398140997 |
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