| Year | Value | | 2020 | 4.028 | | 2019 | 3.948 | | 2018 | 3.931 | | 2017 | 3.939 | | 2016 | 4.127 | | 2015 | 3.909 | | 2014 | 3.914 | | 2013 | 3.878 | | 2012 | 3.831 | | 2011 | 3.861 | | 2010 | 3.839 | | 2009 | 3.698 | | 2008 | 3.639 | | 2007 | 3.585 | | 2006 | 3.635 | | 2005 | 3.546 | | 2004 | 3.528 | | 2003 | 3.543 | | 2002 | 3.498 | | 2001 | 3.356 | | 2000 | 3.213 | | 1999 | 3.314 | | 1998 | 3.437 | | 1997 | 3.287 | | 1996 | 3.589 | | 1995 | 3.659 | | 1994 | 3.637 | | 1993 | 3.797 | | 1992 | 3.89 | | 1991 | 3.797 | | 1990 | 3.93 |
|