| Year | Value | | 2014 | 5.66 | | 2013 | 5.83 | | 2012 | 6.23 | | 2011 | 6.19 | | 2010 | 6.18 | | 2009 | 6.15 | | 2008 | 6.11 | | 2007 | 6.12 | | 2006 | 6.27 | | 2005 | 6.39 | | 2004 | 6.41 | | 2003 | 6.67 | | 2002 | 6.75 | | 2001 | 6.9 | | 2000 | 7.08 | | 1999 | 8.1899995804 | | 1998 | 11.2299995422 | | 1997 | 11.6199998856 | | 1996 | 12.1300001144 | | 1995 | 12.2100000381 | | 1994 | 12.2215003967 | | 1993 | 12.4827003479 | | 1992 | 12.7903003693 | | 1991 | 13.1162004471 | | 1990 | 13.1478004456 | | 1989 | 12.727499961853 | | 1988 | 12.763699531555 | | 1987 | 12.66759967804 | | 1986 | 12.435500144958 | | 1985 | 12.220499992371 | | 1984 | 12.143500328064 | | 1983 | 12.0797996521 | | 1982 | 12.051099777222 | | 1981 | 12.082099914551 | | 1980 | 12.054800033569 |
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