| Year | Value | | 2022 | 7.9675736161788 | | 2021 | 3.4669258531315 | | 2020 | 1.9369412669422 | | 2019 | 2.2060730578152 | | 2018 | 2.4503624385018 | | 2017 | 2.2542765193481 | | 2016 | 1.6055391741421 | | 2015 | 1.4438571927081 | | 2014 | 2.3544905282036 | | 2013 | 2.6516734291658 | | 2012 | 3.7253266611065 | | 2011 | 4.8223963601332 | | 2010 | 3.3263446336864 | | 2009 | 2.8604485590267 | | 2008 | 8.9499533535339 | | 2007 | 4.8102370434291 | | 2006 | 4.2671746339026 | | 2005 | 4.1072507071594 | | 2004 | 3.517999030996 | | 2003 | 3.0250452625224 | | 2002 | 2.9079985703221 | | 2001 | 3.8365726205254 | | 2000 | 3.4335156341877 | | 1999 | 3.0419466709453 | | 1998 | 5.0972914933144 | | 1997 | 5.554129889494 | | 1996 | 6.5260956937258 | | 1995 | 9.0773809523809 | | 1994 | 10.247935555612 | | 1993 | 7.1445870687626 | | 1992 | 7.6361085233697 | | 1991 | 8.9969387349433 | | 1990 | 8.0634609093883 | | 1989 | 6.9239050371124 | | 1988 | 7.1134065293135 | | 1987 | 5.7101193848668 | | 1986 | 5.822666961752 | | 1985 | 6.8075665576833 | | 1984 | 8.0803201725576 | | 1983 | 8.6692715980852 | | 1982 | 10.221727137689 | | 1981 | 12.442436893618 |
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