| Year | Value | | 2015 | 57.666705476787 | | 2014 | 55.60738232183 | | 2013 | 54.576492132248 | | 2012 | 54.903290553105 | | 2011 | 56.766519648514 | | 2010 | 57.002569410619 | | 2009 | 55.689957292194 | | 2008 | 60.341428448149 | | 2007 | 61.746331994806 | | 2006 | 62.176713380294 | | 2005 | 62.360689954961 | | 2004 | 60.734957830439 | | 2003 | 60.166187729532 | | 2002 | 56.257464237769 | | 2001 | 55.848269943518 | | 2000 | 53.525021511816 | | 1999 | 53.254589588625 | | 1998 | 51.482228636455 | | 1997 | 48.761402724373 | | 1996 | 48.573755592185 | | 1995 | 46.269830866571 | | 1994 | 46.633040659405 | | 1993 | 46.704848897108 | | 1992 | 44.618366050493 | | 1991 | 47.31799676927 | | 1990 | 48.972031557874 | | 1989 | 46.735528317822 | | 1988 | 46.041076658308 | | 1987 | 45.331695595614 | | 1986 | 45.739911600118 | | 1985 | 44.855917054059 | | 1984 | 45.739411066638 | | 1983 | 45.702354770023 | | 1982 | 47.031337562918 | | 1981 | 48.813259114701 | | 1980 | 48.881577830951 | | 1979 | 48.099343807861 | | 1978 | 46.270276252345 | | 1977 | 45.667113263541 | | 1976 | 44.952933981837 | | 1975 | 45.276259426963 | | 1974 | 42.21111612233 | | 1973 | 40.310599555182 | | 1972 | 39.727668380725 | | 1971 | 37.765568200393 | | 1970 | 34.185979993143 | | 1969 | 33.323720920514 | | 1968 | 30.72410172477 | | 1967 | 32.751491457615 | | 1966 | 29.730809852967 | | 1965 | 28.142866564665 |
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