| Year | Value | | 2014 | 45.768003082992 | | 2013 | 45.3248866 | | 2012 | 41.801915679909 | | 2011 | 40.923273379314 | | 2010 | 41.708647365496 | | 2009 | 38.541605399548 | | 2008 | 42.719378256961 | | 2007 | 42.713274823656 | | 2006 | 44.53711957709 | | 2005 | 44.931433655714 | | 2004 | 46.162087307456 | | 2003 | 47.086914158265 | | 2002 | 48.206693529935 | | 2001 | 52.029448462618 | | 2000 | 51.116446549813 | | 1999 | 51.721340303392 | | 1998 | 54.177612037512 | | 1997 | 56.043524334502 | | 1996 | 54.50075688106 | | 1995 | 52.947519691905 | | 1994 | 48.666992356529 | | 1993 | 45.109551896727 | | 1992 | 43.82785686798 | | 1991 | 41.336050954028 | | 1990 | 40.002564346657 | | 1989 | 38.669996080423 | | 1988 | 37.227500561197 | | 1987 | 35.876248347528 | | 1986 | 31.025352178593 | | 1985 | 30.87878965646 | | 1984 | 29.437029282877 | | 1983 | 39.061368347878 | | 1982 | 39.291548215762 | | 1981 | 42.917594378036 | | 1980 | 45.687484454973 | | 1979 | 47.699576167786 | | 1978 | 53.725736032563 | | 1977 | 54.50344672378 | | 1976 | 53.630638003972 | | 1975 | 53.070349336485 | | 1974 | 51.670136273874 | | 1973 | 53.190777196858 | | 1972 | 50.29872999375 | | 1971 | 51.133508375714 |
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