| Year | Value | | 2014 | 3.49 | | 2013 | 3.5 | | 2012 | 3.5 | | 2011 | 3.46 | | 2010 | 3.4 | | 2009 | 3.33 | | 2008 | 3.35 | | 2007 | 3.39 | | 2006 | 3.32 | | 2005 | 3.12 | | 2004 | 3.05 | | 2003 | 3.17 | | 2002 | 3.21 | | 2001 | 3.24 | | 2000 | 3.38 | | 1999 | 3.7999999523 | | 1998 | 3.8199999332 | | 1991 | 4.3779997826 | | 1990 | 4.4590997696 | | 1989 | 4.498300075531 | | 1988 | 4.5781998634338 | | 1987 | 4.5935001373291 | | 1986 | 4.7420001029968 | | 1985 | 4.7181000709534 | | 1984 | 4.8137001991272 | | 1983 | 4.9046998023987 | | 1982 | 4.9179000854492 | | 1981 | 4.9228000640869 | | 1980 | 4.83109998703 |
|