| Year | Value | | 2021 | 42.30513544956 | | 2020 | 42.30513544956 | | 2019 | 42.24696692704 | | 2018 | 42.24696692704 | | 2017 | 42.222037560246 | | 2016 | 42.155559248795 | | 2015 | 42.155559248795 | | 2014 | 42.155559248795 | | 2013 | 42.155559248795 | | 2012 | 42.155559248795 | | 2011 | 42.138108692039 | | 2010 | 41.964434103374 | | 2009 | 42.288515871697 | | 2008 | 42.645836795745 | | 2007 | 43.003157719794 | | 2006 | 43.277380754529 | | 2005 | 43.626391889646 | | 2004 | 43.418647166362 | | 2003 | 43.352168854911 | | 2002 | 43.202592654147 | | 2001 | 43.003157719794 | | 2000 | 42.720624896128 | | 1999 | 41.69852085757 | | 1998 | 40.825993019777 | | 1997 | 39.953465181984 | | 1996 | 38.673757686555 | | 1995 | 37.834469004487 | | 1994 | 35.067309290344 | | 1993 | 34.818015622403 | | 1992 | 34.236330397208 | | 1991 | 33.737743061326 | | 1990 | 33.446900448729 | | 1989 | 33.180987202925 | | 1988 | 32.740568389563 | | 1987 | 32.316769154064 | | 1986 | 32.05085590826 | | 1985 | 32.034236330397 | | 1984 | 32.266910420475 | | 1983 | 31.826491607113 | | 1982 | 31.818181818182 | | 1981 | 31.801562240319 | | 1980 | 31.120159547948 | | 1979 | 31.078610603291 | | 1978 | 31.037061658634 | | 1977 | 30.995512713977 | | 1976 | 30.164533820841 | | 1975 | 30.164533820841 | | 1974 | 30.164533820841 | | 1973 | 30.081435931527 | | 1972 | 29.998338042214 | | 1971 | 29.956789097557 | | 1970 | 29.79059331893 | | 1969 | 29.79059331893 | | 1968 | 29.749044374273 | | 1967 | 29.749044374273 | | 1966 | 29.749044374273 | | 1965 | 29.233837460528 | | 1964 | 28.984543792588 | | 1963 | 28.81834801396 | | 1962 | 28.502576034569 | | 1961 | 28.502576034569 |
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