| Year | Value | | 2013 | 25.209949562305 | | 2012 | 24.700742411207 | | 2011 | 24.563959778271 | | 2010 | 24.816959413857 | | 2009 | 23.393973199328 | | 2008 | 20.383408037295 | | 2007 | 21.986997531652 | | 2006 | 22.234772961456 | | 2005 | 21.602120138239 | | 2004 | 22.144457392697 | | 2003 | 21.038440966157 | | 2002 | 18.692477319178 | | 2001 | 20.269023317928 | | 2000 | 16.983823012575 | | 1999 | 13.470188592442 | | 1998 | 11.280167844337 | | 1997 | 11.34521767381 | | 1996 | 11.241832874463 | | 1995 | 11.404999017517 | | 1994 | 13.161949040922 | | 1993 | 13.622508120337 | | 1992 | 15.831219313415 | | 1991 | 17.751791307831 | | 1990 | 24.480685142372 |
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