| Year | Value | | 2017 | 5.5597202504881 | | 2016 | 3.2190585634072 | | 2015 | 5.4289051007927 | | 2014 | 4.9680175234535 | | 2013 | 5.5752253024576 | | 2012 | 4.6606228148247 | | 2011 | 2.0871424859747 | | 2010 | 6.7094774016695 | | 2009 | 7.2361408951629 | | 2008 | 8.240213627428 | | 2007 | 13.573064768972 | | 2006 | 8.1592307129842 | | 2005 | 3.8322964135184 | | 2004 | 8.1249264563588 | | 2003 | 11.846799034118 | | 2002 | 9.5217406253018 | | 2001 | 8.4932786871828 | | 2000 | 10.651946805033 | | 1999 | 11.269949130234 | | 1998 | 12.426012557073 | | 1997 | 9.7844542139105 | | 1996 | 10.604442715766 | | 1995 | 9.0734127247645 | | 1994 | 15.144795086203 | | 1993 | 13.208673285621 | | 1992 | 14.513146269801 | | 1991 | 13.745983854411 | | 1990 | 15.28487370619 | | 1989 | 31.411564405223 | | 1988 | 31.446424368109 | | 1987 | 26.67537020598 | | 1986 | 28.533667314455 |
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