| Year | Value | | 2021 | 38.642664539896 | | 2020 | 38.562910637651 | | 2019 | 39.28829136759 | | 2018 | 39.869355512514 | | 2017 | 40.423835023357 | | 2016 | 40.309900877293 | | 2015 | 40.682085754434 | | 2014 | 42.216398921423 | | 2013 | 42.178420872736 | | 2012 | 42.571113896168 | | 2011 | 43.184839162964 | | 2010 | 43.325357943109 | | 2009 | 43.636777942349 | | 2008 | 43.19623257757 | | 2007 | 43.625384527743 | | 2006 | 43.200030382439 | | 2005 | 44.47989062322 | | 2004 | 45.155899889864 | | 2003 | 45.034370134062 | | 2002 | 45.66634195435 | | 2001 | 46.265167673085 | | 2000 | 46.863819832137 | | 1999 | 47.462477308116 | | 1998 | 48.061219475143 | | 1997 | 48.659988986366 | | 1996 | 49.258788120466 | | 1995 | 49.857828415176 | | 1994 | 50.456845163496 | | 1993 | 51.05592077779 | | 1992 | 51.654908283012 | | 1991 | 52.252414644336 | | 1990 | 52.854050358893 | | 1989 | 53.550947552315 | | 1988 | 53.890851088071 | | 1987 | 54.209866697049 | | 1986 | 54.64661425696 | | 1985 | 54.661805476435 | | 1984 | 54.965629865938 | | 1983 | 55.415669742889 | | 1982 | 55.523907181649 | | 1981 | 57.679161444685 | | 1980 | 55.413770840454 | | 1979 | 54.851695719874 | | 1978 | 55.06019520717 | | 1977 | 54.21822186776 | | 1976 | 54.511792184118 | | 1975 | 52.954692187915 | | 1974 | 51.184915119061 | | 1973 | 51.200676009267 | | 1972 | 51.216436899472 | | 1971 | 49.698074512931 | | 1970 | 49.705670122669 | | 1969 | 49.717063537276 | | 1968 | 50.313318901675 | | 1967 | 50.309521096806 | | 1966 | 50.256351828643 | | 1965 | 50.32851012115 | | 1964 | 50.571569632752 | | 1963 | 50.465231096426 | | 1962 | 50.313318901675 | | 1961 | 49.990505487828 |
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