| Year | Value | | 2022 | 9.9504389471164 | | 2021 | 5.0358076816532 | | 2020 | 4.3329687471169 | | 2019 | 2.8027030792811 | | 2018 | 3.4394690299759 | | 2017 | 8.1568632340337 | | 2016 | 5.7063750461621 | | 2015 | 4.3771266506565 | | 2014 | 3.7405295157197 | | 2013 | 5.6116293381582 | | 2012 | 6.8317870341855 | | 2011 | 7.6228226284852 | | 2010 | 4.3974138315007 | | 2009 | 5.0062736153405 | | 2008 | 11.677982715495 | | 2007 | 7.9286638492638 | | 2006 | 7.3106761356186 | | 2005 | 8.4487264229953 | | 2004 | 3.9943840081597 | | 2003 | 6.4894320867334 | | 2002 | 3.3001217528375 | | 2001 | 4.2490551879622 | | 2000 | 3.6073112329965 | | 1999 | 1.666105324261 | | 1998 | 4.8161746093202 | | 1997 | 5.0293198643137 | | 1996 | 7.8306309172108 | | 1995 | 16.433503452416 | | 1994 | 25.177876224996 | | 1993 | 9.6793465806793 | | 1992 | 9.5604118760082 | | 1991 | 8.9969387349433 | | 1990 | 6.0773254041294 | | 1989 | 8.0461586348732 | | 1988 | 11.690586887676 | | 1987 | 5.6231262738223 | | 1986 | 3.1848838876498 | | 1985 | 10.471408112053 | | 1984 | 9.5430695745469 | | 1983 | 9.3554650798615 | | 1982 | 12.059765208134 | | 1981 | 12.167394348794 |
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