| Year | Value | | 2023 | 35.437 | | 2022 | 36.045 | | 2021 | 36.342 | | 2020 | 36.601 | | 2019 | 37.138 | | 2018 | 37.624 | | 2017 | 38.171 | | 2016 | 38.732 | | 2015 | 39.364 | | 2014 | 39.646 | | 2013 | 40.288 | | 2012 | 40.702 | | 2011 | 40.973 | | 2010 | 41.759 | | 2009 | 42.162 | | 2008 | 42.538 | | 2007 | 43.831 | | 2006 | 44.087 | | 2005 | 44.952 | | 2004 | 46.492 | | 2003 | 47.056 | | 2002 | 47.488 | | 2001 | 49.623 | | 2000 | 51.491 | | 1999 | 50.948 | | 1998 | 51.457 | | 1997 | 52.117 | | 1996 | 52.491 | | 1995 | 52.683 | | 1994 | 52.41 | | 1993 | 50.834 | | 1992 | 50.315 | | 1991 | 52.279 | | 1990 | 52.888 | | 1989 | 51.682 | | 1988 | 51.708 | | 1987 | 52.222 | | 1986 | 52.195 | | 1985 | 51.693 | | 1984 | 50.351 | | 1983 | 51.173 | | 1982 | 51.973 | | 1981 | 55.185 | | 1980 | 53.452 | | 1979 | 52.029 | | 1978 | 51.288 | | 1977 | 51.331 | | 1976 | 51.329 | | 1975 | 51.321 |
|