| Year | Value | | 2013 | 2.89 | | 2012 | 2.88 | | 2011 | 2.88 | | 2010 | 2.99 | | 2009 | 3.01 | | 2007 | 3.09 | | 2006 | 3.12 | | 2005 | 3.08 | | 2004 | 3.01 | | 2003 | 3.07 | | 2002 | 3.14 | | 2001 | 3.26 | | 2000 | 3.26 | | 1999 | 3.0299999714 | | 1998 | 3.0499999523 | | 1997 | 3.0499999523 | | 1996 | 3.1400001049 | | 1995 | 3.1900000572 | | 1994 | 3.0171000957 | | 1993 | 3.8313999176 | | 1992 | 4.0134000778 | | 1991 | 3.9986999035 | | 1990 | 4.0248999596 | | 1989 | 4.1321001052856 | | 1988 | 3.9354999065399 | | 1987 | 3.9700000286102 | | 1986 | 4.0696997642517 | | 1985 | 4.1388001441956 | | 1984 | 4.1389999389648 | | 1983 | 4.0862002372742 | | 1982 | 4.1606998443603 | | 1981 | 4.1870999336243 | | 1980 | 4.2716999053955 | | 1960 | 5.1026759147644 |
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