| Year | Value | | 2015 | 45.333785025636 | | 2014 | 46.197275514046 | | 2013 | 45.30505616412 | | 2012 | 49.01078210859 | | 2011 | 51.154918072064 | | 2010 | 52.235433931489 | | 2009 | 50.279074231582 | | 2008 | 53.353573214388 | | 2007 | 56.167149150587 | | 2006 | 51.324112197962 | | 2005 | 51.459986970515 | | 2004 | 57.412209372385 | | 2003 | 56.808486917946 | | 2002 | 53.963402654278 | | 2001 | 53.781972210007 | | 2000 | 53.922885851866 | | 1999 | 52.547615848773 | | 1998 | 58.27799167596 | | 1997 | 53.383312314087 | | 1996 | 56.236998808477 | | 1995 | 54.496864710342 | | 1994 | 57.2256188602 | | 1993 | 58.49549694153 | | 1992 | 55.321746505943 | | 1991 | 61.206942270726 | | 1990 | 57.432772714993 | | 1989 | 59.214765852565 | | 1988 | 59.468455994701 | | 1987 | 63.635832385253 | | 1986 | 58.01600012461 | | 1985 | 59.835714024155 | | 1984 | 56.07655799387 | | 1983 | 56.54897825192 | | 1982 | 56.477895542887 | | 1981 | 62.157329649221 | | 1980 | 71.898924802784 | | 1979 | 72.801004562086 | | 1978 | 75.908813301097 | | 1977 | 76.374513907705 | | 1976 | 80.262593141195 | | 1975 | 77.822518633564 | | 1974 | 75.754378156555 | | 1973 | 76.781002048912 | | 1972 | 74.901862319418 | | 1971 | 72.57189375021 | | 1970 | 71.9935270349 | | 1969 | 70.338717909068 | | 1968 | 66.36720814401 | | 1967 | 62.364893047114 | | 1966 | 61.103739375104 | | 1965 | 58.693484968353 | | 1964 | 55.789415534649 | | 1963 | 51.937052774554 | | 1962 | 47.591641042049 | | 1961 | 45.280470766316 | | 1960 | 45.61990106968 |
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