| Year | Value | | 2023 | 3.05 | | 2022 | 3.01 | | 2021 | 2.94 | | 2020 | 2.92 | | 2019 | 2.78 | | 2018 | 2.75 | | 2017 | 2.72 | | 2016 | 2.67 | | 2015 | 2.62 | | 2014 | 2.65 | | 2013 | 2.64 | | 2012 | 2.66 | | 2011 | 2.62 | | 2010 | 2.73 | | 2009 | 2.6 | | 2008 | 2.56 | | 2007 | 2.52 | | 2006 | 2.49 | | 2005 | 2.47 | | 2004 | 2.43 | | 2003 | 2.44 | | 2002 | 2.45 | | 1999 | 2.5999999046 | | 1998 | 2.5 | | 1997 | 2.5 | | 1996 | 2.5 | | 1995 | 2.5 | | 1994 | 2.5 | | 1993 | 2.5 | | 1992 | 2.4000000954 | | 1991 | 2.4000000954 | | 1990 | 2.4000000954 | | 1989 | 2.0999999046326 | | 1988 | 2.0999999046326 | | 1987 | 2.0999999046326 | | 1986 | 2.0999999046326 | | 1985 | 2.0999999046326 | | 1980 | 2.2000000476837 |
|