| Year | Value | | 2014 | 2.99 | | 2013 | 3.495 | | 2012 | 3.495 | | 2011 | 3.492 | | 2010 | 3.491 | | 2009 | 3.499 | | 2008 | 3.124 | | 2007 | 3.092 | | 2006 | 3.085 | | 2005 | 3.017 | | 2004 | 3.013 | | 2003 | 3.008 | | 2002 | 3.019 | | 2001 | 3.027 | | 2000 | 3.027 | | 1999 | 2.986 | | 1998 | 2.99 | | 1997 | 2.97 | | 1996 | 2.992 | | 1995 | 4.407 | | 1994 | 4.314 | | 1993 | 4.342 | | 1992 | 4.297 | | 1991 | 4.345 | | 1990 | 4.29 | | 1989 | 4.275 | | 1988 | 4.2 | | 1987 | 4.143 | | 1986 | 4.06 | | 1985 | 4.002 | | 1984 | 3.909 | | 1983 | 3.82 | | 1982 | 3.741 | | 1981 | 3.631 | | 1980 | 3.515 |
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