| Year | Value | | 2021 | 12.75 | | 2020 | 12.65 | | 2019 | 12.42 | | 2018 | 12.42 | | 2017 | 12.25 | | 2016 | 11.97 | | 2015 | 11.61 | | 2014 | 11.63 | | 2013 | 10.99 | | 2012 | 10.37 | | 2011 | 9.68 | | 2010 | 8.87 | | 2009 | 8.31 | | 2008 | 7.83 | | 2007 | 7.31 | | 2006 | 6.54 | | 2005 | 5.93 | | 2004 | 5.46 | | 2003 | 5.12 | | 2002 | 4.87 | | 2001 | 4.48 | | 2000 | 4.67 | | 1999 | 5.5999999046 | | 1998 | 5.0999999046 | | 1997 | 4.8000001907 | | 1996 | 4.5999999046 | | 1995 | 4.4000000954 | | 1994 | 4.0999999046 | | 1993 | 3.7000000477 | | 1992 | 3.5 | | 1991 | 3.2999999523 | | 1990 | 3.0999999046 | | 1989 | 3 | | 1985 | 2.4000000953674 | | 1980 | 1.7000000476837 |
|