| Year | Value | | 2022 | 42.253515954914 | | 2021 | 41.803485163784 | | 2020 | 41.31819116772 | | 2019 | 40.84185096987 | | 2018 | 40.371914952576 | | 2017 | 39.909984462244 | | 2016 | 39.456412384497 | | 2015 | 39.002837102218 | | 2014 | 38.539542378091 | | 2013 | 38.07627699892 | | 2012 | 37.634505256316 | | 2011 | 37.214488415675 | | 2010 | 36.797138967469 | | 2009 | 36.387235900369 | | 2008 | 35.982922438583 | | 2007 | 35.583753972808 | | 2006 | 35.18492421948 | | 2005 | 34.790476815488 | | 2004 | 34.405228740504 | | 2003 | 34.030230885188 | | 2002 | 33.667559306345 | | 2001 | 33.313508214019 | | 2000 | 33.000642094968 | | 1999 | 32.737408161048 | | 1998 | 32.483275652223 | | 1997 | 32.22877124406 | | 1996 | 31.975922658849 | | 1995 | 31.727233714731 | | 1994 | 31.483627612441 | | 1993 | 31.240014787772 | | 1992 | 30.991062712326 | | 1991 | 30.72281567083 | | 1990 | 30.43481906395 | | 1989 | 30.092777443265 | | 1988 | 29.755868898077 | | 1987 | 29.413247312458 | | 1986 | 29.068129625458 | | 1985 | 28.718369066081 | | 1984 | 28.374309320419 | | 1983 | 28.036431312833 | | 1982 | 27.708436732748 | | 1981 | 27.379688704396 | | 1980 | 26.959453336895 | | 1979 | 26.533010366536 | | 1978 | 26.132739273466 | | 1977 | 25.724667472431 | | 1976 | 25.299527281137 | | 1975 | 24.868122692175 | | 1974 | 24.595121496245 | | 1973 | 24.196223086776 | | 1972 | 23.80708191087 | | 1971 | 23.410976546585 | | 1970 | 23.065567989398 | | 1969 | 22.758726544133 | | 1968 | 22.453312807819 | | 1967 | 22.15024107666 | | 1966 | 21.854044090872 | | 1965 | 21.560221622955 | | 1964 | 21.264711635814 | | 1963 | 20.975826697003 | | 1962 | 20.691442998104 | | 1961 | 20.409973963686 | | 1960 | 20.162105653106 |
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