| Year | Value | | 2021 | 41.247263652798 | | 2020 | 41.19411907693 | | 2019 | 41.115023341898 | | 2018 | 41.045971812785 | | 2017 | 40.893923576345 | | 2016 | 40.763819760058 | | 2015 | 40.648652863256 | | 2014 | 40.499144752967 | | 2013 | 40.38168312602 | | 2012 | 40.163705489084 | | 2011 | 40.111551175882 | | 2010 | 39.954958099688 | | 2009 | 39.761317461499 | | 2008 | 39.537661498309 | | 2007 | 39.417351810964 | | 2006 | 39.215697477571 | | 2005 | 39.051814250167 | | 2004 | 38.628689300932 | | 2003 | 38.816320090338 | | 2002 | 38.518851152739 | | 2001 | 38.406830397158 | | 2000 | 38.216097128846 | | 1999 | 38.068508243764 | | 1998 | 37.868966006935 | | 1997 | 37.780476110105 | | 1996 | 37.59298078529 | | 1995 | 37.395563748136 | | 1994 | 37.169527494002 | | 1993 | 36.933567981597 | | 1992 | 37.555224371029 | | 1991 | 37.327993950727 | | 1990 | 37.234248385722 | | 1989 | 37.15934491318 | | 1988 | 37.034593375285 | | 1987 | 36.794019822864 | | 1986 | 36.667423362837 | | 1985 | 36.449670097981 | | 1984 | 36.249888608775 | | 1983 | 36.214140643555 | | 1982 | 36.164390458267 | | 1981 | 36.130185311237 | | 1980 | 36.071344801391 | | 1979 | 36.024563391155 | | 1978 | 36.033775957636 | | 1977 | 35.952657202855 | | 1976 | 35.931411236311 | | 1975 | 35.880044878729 | | 1974 | 35.84397603051 | | 1973 | 35.847435132156 | | 1972 | 35.828578807998 | | 1971 | 35.740164925208 | | 1970 | 35.697375191721 | | 1969 | 35.674128009379 | | 1968 | 35.636634158072 | | 1967 | 35.573481384651 | | 1966 | 35.536285892904 | | 1965 | 35.528627997544 | | 1964 | 35.493670202493 | | 1963 | 35.439965481788 | | 1962 | 35.328627639512 | | 1961 | 35.267165570262 |
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