| Year | Value | | 2014 | 4.03 | | 2013 | 3.93 | | 2012 | 3.98 | | 2011 | 4.07 | | 2010 | 4.1 | | 2009 | 4.02 | | 2008 | 4.04 | | 2007 | 4.07 | | 2006 | 4.34 | | 2005 | 4.4 | | 2004 | 4.9 | | 2003 | 5.08 | | 2002 | 5.02 | | 2001 | 5.13 | | 2000 | 5.36 | | 1997 | 7.1100001335 | | 1996 | 8.0100002289 | | 1995 | 10.2600002289 | | 1994 | 11.470700264 | | 1993 | 11.678899765 | | 1992 | 11.4153003693 | | 1991 | 11.5550003052 | | 1990 | 11.4849996567 | | 1989 | 11.193400382996 | | 1988 | 11.129300117493 | | 1987 | 11.104999542236 | | 1986 | 11.122200012207 | | 1985 | 10.668000221252 | | 1980 | 10.460499763489 |
|