| Year | Value | | 2014 | 41.568151741425 | | 2013 | 42.473182982132 | | 2012 | 40.214532863582 | | 2011 | 41.653678010729 | | 2010 | 40.106779445485 | | 2009 | 39.998087302656 | | 2008 | 39.407486596368 | | 2007 | 42.521397876852 | | 2006 | 43.20148309173 | | 2005 | 44.257381326574 | | 2004 | 46.761410484467 | | 2003 | 44.65322967811 | | 2002 | 44.112296333411 | | 2001 | 42.031732496566 | | 2000 | 39.202479517943 | | 1999 | 41.161409291284 | | 1998 | 39.98009892472 | | 1997 | 43.239135869897 | | 1996 | 46.345821974758 | | 1995 | 46.397163088041 | | 1994 | 44.033970772453 | | 1993 | 41.000518666638 | | 1992 | 36.420695792704 | | 1991 | 34.967711804861 | | 1990 | 36.639938325191 | | 1989 | 33.55939974294 | | 1988 | 34.387881348408 | | 1987 | 32.409592072766 | | 1986 | 32.617204922597 | | 1985 | 33.098252081572 | | 1984 | 42.390232304429 | | 1983 | 45.796257776132 | | 1982 | 39.875966741968 | | 1981 | 45.621446463227 | | 1980 | 49.175416770748 | | 1979 | 46.768997552835 | | 1978 | 48.192162581227 | | 1977 | 45.057425847438 | | 1976 | 43.686483590784 | | 1975 | 41.675511086762 | | 1974 | 44.948451344713 | | 1973 | 47.697059988516 | | 1972 | 44.54877951352 | | 1971 | 42.34296517285 |
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