| Year | Value | | 2014 | 3.966891088245 | | 2013 | 4.0795274042676 | | 2012 | 4.5738842765606 | | 2011 | 4.926461702594 | | 2010 | 4.685557034042 | | 2009 | 5.6536484971796 | | 2008 | 4.4458294982373 | | 2007 | 4.8507234872252 | | 2006 | 4.3084668349406 | | 2005 | 4.8683170115889 | | 2004 | 5.301366496456 | | 2003 | 6.854552052015 | | 2002 | 6.8956079503431 | | 2001 | 5.4810146916463 | | 2000 | 4.9153923650994 | | 1999 | 5.1727224363936 | | 1998 | 6.0815687041383 | | 1997 | 5.8057525967211 | | 1996 | 5.2813459409681 | | 1995 | 6.3505784830546 | | 1994 | 8.3987590412286 | | 1993 | 7.0639204164576 | | 1992 | 7.2770431912171 | | 1991 | 7.0112188585833 | | 1990 | 6.5511834439503 |
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