| Year | Value | | 2011 | 28.775905810109 | | 2010 | 23.344490411443 | | 2009 | 11.95573112666 | | 2008 | 21.243592449265 | | 2007 | 28.459237581161 | | 2006 | 25.224705484078 | | 2005 | 16.11373274054 | | 2004 | 25.489269013642 | | 2003 | 28.513212992026 | | 2002 | 33.942992621666 | | 2001 | 39.276873518386 | | 2000 | 40.456005424782 | | 1999 | 38.952516923859 | | 1998 | 41.932933239494 |
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