| Year | Value | | 2021 | 40.303463945032 | | 2020 | 40.260934954953 | | 2019 | 40.191191437498 | | 2018 | 40.133367904747 | | 2017 | 40.036187115954 | | 2016 | 39.903621070478 | | 2015 | 39.770792178877 | | 2014 | 39.660517208023 | | 2013 | 39.539573839373 | | 2012 | 39.339988417546 | | 2011 | 39.292127971935 | | 2010 | 39.114494194969 | | 2009 | 38.913404122357 | | 2008 | 38.675793329497 | | 2007 | 38.541828653778 | | 2006 | 38.364198050175 | | 2005 | 38.200955798987 | | 2004 | 37.807571426168 | | 2003 | 37.986098027485 | | 2002 | 37.673704256504 | | 2001 | 37.574312628747 | | 2000 | 37.398434393422 | | 1999 | 37.219507213292 | | 1998 | 37.019890622459 | | 1997 | 36.90109824209 | | 1996 | 36.723677637594 | | 1995 | 36.545887108438 | | 1994 | 36.36353378761 | | 1993 | 36.153039964191 | | 1992 | 36.820762030909 | | 1991 | 36.718888169918 | | 1990 | 36.63196237318 | | 1989 | 36.567017110198 | | 1988 | 36.442221518483 | | 1987 | 36.21764309474 | | 1986 | 36.10407245078 | | 1985 | 35.894990415087 | | 1984 | 35.712778718929 | | 1983 | 35.706093418187 | | 1982 | 35.654259986876 | | 1981 | 35.631929294352 | | 1980 | 35.585489798112 | | 1979 | 35.543967425003 | | 1978 | 35.545358126495 | | 1977 | 35.475127701165 | | 1976 | 35.466386148931 | | 1975 | 35.438785691112 | | 1974 | 35.394921080252 | | 1973 | 35.411745020642 | | 1972 | 35.417446670207 | | 1971 | 35.368029062917 | | 1970 | 35.332964911413 | | 1969 | 35.312353547568 | | 1968 | 35.278382046678 | | 1967 | 35.208204077732 | | 1966 | 35.177063535249 | | 1965 | 35.142098715619 | | 1964 | 35.089800483985 | | 1963 | 35.034820291754 | | 1962 | 34.947010915182 | | 1961 | 34.896401325405 |
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