| Year | Value | | 2023 | 35.209 | | 2022 | 35.407 | | 2021 | 35.668 | | 2020 | 35.895 | | 2019 | 35.947 | | 2018 | 36.046 | | 2017 | 36.067 | | 2016 | 36.034 | | 2015 | 35.938 | | 2014 | 35.738 | | 2013 | 35.483 | | 2012 | 35.166 | | 2011 | 34.708 | | 2010 | 35.609 | | 2009 | 36.561 | | 2008 | 36.918 | | 2007 | 37.529 | | 2006 | 38.152 | | 2005 | 38.39 | | 2004 | 38.911 | | 2003 | 39.37 | | 2002 | 39.579 | | 2001 | 40.164 | | 2000 | 40.859 | | 1999 | 41.474 | | 1998 | 41.872 | | 1997 | 42.638 | | 1996 | 43.189 | | 1995 | 45.894 | | 1994 | 47.342 | | 1993 | 48.571 | | 1992 | 49.48 | | 1991 | 50.222 | | 1990 | 50.882 | | 1989 | 51.399 | | 1988 | 52.09 | | 1987 | 52.537 | | 1986 | 53.111 | | 1985 | 53.567 | | 1984 | 53.876 | | 1983 | 54.255 | | 1982 | 54.452 | | 1981 | 54.647 | | 1980 | 54.878 | | 1979 | 54.898 | | 1978 | 54.244 | | 1977 | 54.384 | | 1976 | 54.56 | | 1975 | 54.72 |
|